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Each Purana has a particular goal or intention; a primary thing that it tries to convey.

Ramanujacharya in the Vedartha Sangraha says:

All other Puranas with other aims must be interpreted consistently
with this Purana. That they have other aims is discerned in the manner
of their commencement.

So, it is agreed that different Puranas have different aims.

For example, according to the Garuda Purana:

3-5. Those sages once, in the morning,. having offered oblations to
the sacrificial fire respectfully asked this of the revered Sūta
sitting there:--

The sages said: The happiness-giving path of the Shining Ones has been
described by you. We now wish to hear about the fear-inspiring Way
of Yama;

What is the goal of the Shiva Purana?

What is the goal of the Shiva Purana??

The goal of the Shiva Purana is to guide people away from Tamas.

The introduction of the Shiva Purana clarifies the intent and target audience of this Purana:

2. How do good conduct, good devotion and power of discrimination flourish? How are base feelings [tamasic feelings] dispelled by good men?




3. In this terrible Kali age all living beings have almost become demoniac in character. What is the effective mode of remedying the
   same?

...

27. For the benefit of the evil-minded persons of the Kali age, bereft of virtuous conduct, Lord Śiva has produced the nectar in the form of Śivapurāṇa.

Look at the words used: "good conduct", "base feelings", "demonic character", "discrimination between good and evil", etc.

Bad conduct, base feelings, demonic character, and lack of discrimination between right and wrong are all tamasic qualities, and this Purana clearly says that its intent is to remove these qualities from bad men.

The Manusmriti describes some qualities arising out of Tamas:

12.33 - Avarice, drowsiness, irresolution, cruelty, disbelief, bad character, habit of begging, and inattentiveness are the characteristics of the quality of ‘Tamas.’

Since the primary goal of this Purana is to remove Tamasic qualities from people, it is described as a Tamasic Purana and is for Tamasic people:

4. Who are those among sinners in the Kali age who get sanctified by this story? Please enlighten us. Make the whole world gratified.




5. Men who habitually commit sins, wicked persons indulging in vicious activities and persons of lecherous disposition become pure hereby.




6. This is a great Jñānayajña (sacrificial rite of wisdom); it yields worldly enjoyment as well as salvation; it dispels all sins and
   delights Siva.




7. Men overwhelmed by the thirst of covetousness, those devoid of truthfulness, those who decry even their parents, haughty vain fellows
   and persons prone to violent activities become sanctified by this.

...

12. Men of unclean habits and wicked minds, men who know no peace and men who swallow temple and trust properties become sanctified by this
    Jñānayajña even in the Kali age.

The Shiva Purana even explicitly says that it is for people fallen away from Sattva:

15. But later on as her youthful health and boisterous virility increased, cupid’s onslaught became extremely unbearable for her and she ceased from strictly adhering to her virtuous conduct.




16. Unknown to her husband she began to indulge in sexual intercourse with her sinful paramour at night. Fallen thus from Sāttvic virtues she went ahead along her evil ways.

...

20. The wicked Binduga caught hold of his wife and with threats and abuses fisted her again and again.




21. The whorish wicked woman Cañculā thus beaten by her husband became infuriated and spoke to her wicked husband.




22. Foul-minded that you are, you indulge in sexual intercourse with the harlot every day. You have discarded me your wife, ever ready to
    serve you with my youthful body.




23. I am a youthful maiden endowed with beauty and mentally agitated by lust. Tell me what other course can I take when I am denied the
    amorous sport with my husband.




24. I am very beautiful and agitated with flush of fresh youth. Deprived of sexual intercourse with you I am extremely distressed. How
    can I bear the pangs of passion?

As you can see, words like "wicked", "evil", and "lust" are constantly repeated in every single verse to emphasize the point that these qualities are bad.

After that, the husband even tells his wife to continue:

27. You go ahead with your sexual sports with any number of paramours. No fear need enter your mind. Extract as much of wealth as you can from them and give them enough sexual pleasure.




28. You must hand over all the amount to me. You know that I am enamoured of my concubine. Thus our mutual interests will be assured.




29. His wife Cañculā on hearing these words of her husband became extremely delighted and assented to his vicious proposal.

...

33. The foolish fellow endured distress and torture in Hell for many days. He then became a ghost in the Vindhya mountain range continuing to be terribly sinful.

The woman luckily went to a temple, heard the Shiva Purana, and became good:

36. Due to divine intercession it chanced that on an auspicious occasion she happened to go to the Gokarṇa temple in the company of her kinsmen.




37. Casually moving about here and there with her kinsmen she happened to take her bath in a holy pond as a normal routine affair.




38. In a certain temple a scholar of divine wisdom was conducting a discourse on the holy Śivapurāṇa story some of which she happened to
    hear.

39—40. The portion that fell on her ears was the context in which it
was said that the servants of Yama would introduce a red hot iron into
the vaginal passage of women who indulge in sexual intercourse with
their paramours. This narrative made by the Paurāṇika to increase
detachment, made the woman tremble with fear.

41. At the end of the discourse when all the people dispersed, the terrified woman approached the scholarly brahmin and spoke to him in
    confidence.

The learned scholar says that she should listen to the Shiva Purana, and this is because the woman is very sinful and tamasic:

l-2. O Brahmin lady, fortunately you have realised at the proper time on hearing the story of Śivapurāṇa that is conducive to non-attachment. Do not be afraid. Seek refuge in Śiva. All sins perish instantaneously by Śiva’s grace.

As you can see, the background, context, intent, and target audience of this Purana is different.

Compare the introduction of this Purana to the Vishnu Purana:

May that Vishńu, who is the existent, imperishable, Brahma, who is Íśwara, who is spirit; who with the three qualities 5 is the cause of creation, preservation, and destruction; who is the parent of nature, intellect, and the other ingredients of the universe; be to us the bestower of understanding, wealth, and final emancipation.

Introduction to the Srimad Bhagavata Purana:

I meditate upon Lord Śrī Kṛṣṇa because He is the Absolute Truth and the primeval cause of all causes of the creation, sustenance and destruction of the manifested universes. He is directly and indirectly conscious of all manifestations, and He is independent because there is no other cause beyond Him. It is He only who first imparted the Vedic knowledge unto the heart of Brahmājī, the original living being.

These Sattvic puranas start off with a salutation to Vishnu and describe philosophical concepts; nothing to do with tamasic qualities.

The goal of the Vishnu Purana is to just give general knowledge of the universe:

I am now desirous, oh thou who art profound in piety! to hear from thee, how this world was, and how in future it will be? what is its substance, oh Brahman, and whence proceeded animate and inanimate things? into what has it been resolved, and into what will its dissolution again occur?....

The goal of the Bhagavata Purana is to increase devotion to Vishnu:

This beautiful Bhāgavatam, compiled by the great sage Vyāsadeva, is sufficient in itself for God realization. What is the need of any other scripture? As soon as one attentively and submissively hears the message of Bhāgavatam, by this culture of knowledge the Supreme Lord is established within his heart.

So, the Puranas have different intents and audiences.