I understand that in theory it's more flexible, but I always imagined some sort of standard, where apps and services can expose a set of pre-approved actions on the user's behalf. And the user can add/revoke privileges from agents at any point. Kind of like OAuth scopes.
Imagine having "app stores" where you "install" apps like Gmail or Uber or whatever on your agent of choice, define the privileges you wish the agent to have on those apps, and bam, it now has new capabilities. No browser clicks needed. You can configure it at any time. You can audit when it took action on your behalf. You can see exactly how app devs instructed the agent to use it (hell, you can even customize it). And, it's probably much faster, cheaper, and less brittle (since it doesn't need to understand any pixels).
Seems like better UX to me. But probably more difficult to get app developers on board.