[mmarian]

Data capture.

They can pretend to be humans when requesting content from sites, thus bypassing blocks by Cloudflare et al. The content can be then passed to the AI mothership.

They also get complete visibility over what you do, as @benouau says.

[meerab]

Bingo - that is the real reason!

If an Agentic requests originate from a server - it is easy for receivers to block the server. Write a browser-plugin and you are playing in somebody else's garden.

Write a browser and now you get to pretend like a human!