I find this way of doing it highly annoying when I come back to a site I haven't used in a while. First having to enter my email to get my username and then enter my username *and* email to reset my password.

Why do sites split it up like this? Is it believed to be better UX? Better security?

I mean, the weak point is my email in both cases, so if someone got a hold of that they would get both the username and the password anyways. So why not just combine it into one function where you only need to supply your email?