Just like being "home alone," but with a college dorm room instead. When you walk into the room and find out your roommates are gone, then you know your dorm alone, which gives you a certain amount of privacy and with it freedom to do things that would normally be too awkward or obnoxious with other people around.
Patrick: Back at the dorm. I wonder if my roommates are in?
*opens up door, walks in, and briefly scans the room*
Nope, looks like I'm dorm alone. Hell yeah, now I can play music without headphones/talk to myself out loud/jack off to porn/take a piss on my roommate's bed! 'Tis the Grandest Day!