Remembering the username can be pretty useful, especially if you find yourself in a place where you common usernames are taken on other sites and you have to use different usernames in different places.
Being reminded of a username could also be helpful if you have more
than one username at a given site - for instance, personal and
business accounts for a bank or credit card site, per-project
accounts for a contractor, etc.
Not to mention the sheer convenience of one less thing to type--especially helpful if you are on a mobile device, where typing is still a pain vs. on a full keyboard.