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This has already been asked but the answer doesn't seem to be quite right as it states that "a" is supposed to be used as an "adversative conjunctive" for two parts of a sentence in opposition with each other.

However, on Duolingo I am shown an example where it says "Я Анна, а это — Том" using а for the and when I would expect to use и.

Why is this? Sure this could be in opposition if you are saying no this anna but this is Tom but it's not saying that! It's merely stating there are two people and their names.

Can someone explain why you would use а here?

И joins, unites words and clauses, a contrasts them, singles them out, sets them aside.

Imagine a photo of Anna and Tom, a married couple. When showing the photo to your friend who doesn't know them, you can say:

Это — Анна и Том.

By saying this, you show them as a single entity, a married couple in our case. But you can say it with a, too:

Это — Анна, а это — Том.

Here you just name the two people in the photo one by one making no allusions to whether they are connected in a way or not, a just separates them from each other, just the same way as if you said Это — Анна. Это — Том.

Now imagine that both of them like reggae music. You will tell your friend about it this way:

Анна любит регги, и Том тоже любит регги.

You use и because reggae is something that unites them. But in a similar situation, if there's something that sets them aside, you'll have to use a:

Анна любит кошек, а Том любит собак.

or

Анна всегда ездит на машине, а Том на велосипеде.

Or imagine that you and your friend are in a store, you point to different kinds of candies and explain to your friend which ones you like:

Эти конфеты мне очень нравятся, и эти нравятся, и те тоже, и те, а вон те я не люблю, они слишком кислые.

You like the first four kinds of candies, and that fact unites them, that's why you use и when speaking about them, but then there's the kind you don't like, you set it aside from the previous ones by using a. That's the main principles of how и and a work.

И объединяет, а а разъединяет.

А in this sense is not the adversative conjunctive, it's a topical particle.

It's used in modern Russian to introduce a new topic in questions:

А ты поел?

and to express surprise or announce news:

А Маша замуж вышла!

, but on the earlier stage of the language development it was used to start virtually every sentence on its own, which was not linked to the previous one.

Here's an excerpt from the birch bark gramota 109:

Коупилъ еси робоу плъскове

A ныне мѧ въ томъ ѧла кънѧгыни

А ныне сѧ дроужина по мѧ пороучила

А ныне ка посъли къ томоу моужеви грамотоу ели оу него роба

А се ти хочоу коне коупивъ и кънѧжъ моужъ въсадивъ та на съводы

А ты атче еси не възалъ коунъ техъ а не емли ничъто же оу него

(segmentation mine)

This usage you're talking about is the relic of that extinct usage pattern. Basically, you're telling two equally important things, not depending on each other, and you're prepending the а to the second one:

(А) я — Анна. А он — Том.

If your sentences are linked, you're not introducing a new topic and you should be using и instead:

Я Анна, и я буду сегодня вести вашу экскурсию.

Usage of а as an adversative conjunctive is slightly different:

Я Анна, а не Том. // I'm Anna, not Tom.