Because I don't want to have to write session management shenanigans for every simple server-side data slinger I write. Also, it's a router.
What does it do? Manages sessions, holds onto some state for each session, and also really simple routing.
You can require authentication for some paths, but you'll have to define what authentication means yourself.
Still in development, I'll write up some actual documentation after I have an app or two that's actually using it, at which point I will presumably have a better idea about what the full feature set is going to look like.
Until then, check out example.js for some ideas on what it currently does.