I think you've pretty much summed it up.

As far as I'm aware, it has less functionality than the OEM, so you use it to _remove_ features (good and/or bad).

Aside from that, I suppose it means you can run a more up to date firmware if yours is no longer maintained, but I'm not sure what that means in practical terms.

There's also the "hyper paranoid" fork "canoeboot" which has no proprietary blobs, and presumably _even less_ functionality.

The short answer is; if you don't know why you want it or need it, you probably don't.