You're thinking of this in the classical ad placement sense. If there's an established sense of trust with an AI, advertising can become much more subtle and even more manipulative than it currently is. Ad campaigns can last years, and span many conversations, where brands can be woven into the subconscious much more deeply than a traditional ad could do.
They could then be surfaced whenever the user is most vulnerable to be swayed into making a purchase.
> In your classic scenario, you navigate the SERP and hit various websites, that's a lot of surface area for ads.
Most of which never get seen or clicked on. It's a lot of wasted real estate. Ad conversion can become much more efficient if the system deeply knows the person they're interacting with.