Can anyone explain to me the rationale for the recent trend to split the input of username and password into two steps? I noticed more and more services (includ

Can anyone explain to me the rationale for the recent trend to split the input of username and password into two steps? I noticed more and more services (including Outlook, iCloud, Evernote, etc.) have adopted such an annoying design, which makes no sense to me. I guess the purpose for doing so is to preempt user from typing a wrong/non-exist username so that server resources for checking unmatched credentials are saved and brutal force attacks are prevented? But do they justify all the hassle caused to daily users?