To help Academia.edu grow, of course. Think about it this way:
Sure, at a personal level, it might be slightly annoying to you to have to create an Academia.edu account. But Academia's entire value is in the users, content, and community which have built up around it. If viral hooks like "signup to download" substantially grow the Academia community (and I believe this one does), then the value which they will create for you in having an accessible library of the world's research will far offset the small annoyance of having to sign up.
You help Academia grow by increasing the userbase, forcing users to log in puts up a barrier where none is needed. People come to academia.edu (to which I'm sympathetic, I submitted the link) for the papers, not to make an account.
Don't put yourself before the content, that's the same mistake everybody else is making in this space.
Why?