I think the main driving force is it’s a way to monetize an LLM. If the LLM is doing the buying then a “buyer fee” can be tacked on to the purchase and paid to the LLM provider. That is probably an easier sell than an ongoing monthly subscription.

Also, sellers can offer a payment to the LLM provider to favor their products over competitors.

That's kinda backwards, tho. You don't, or at least shouldn't, say "we have this thing, and we need to make people use it, so let's make up use cases even if they make no sense and will fail". (People do this all the time, of course; it's more or less the sunk-cost fallacy.)

It will certainly happen but it seems like a shady kickback unseen by the the end-user is well beyond relevant ads colocated with search results.

Seems like something that should really be illegal, unless the ads are obvious.