Harmless fun. Sort of like Trekkies spending their days drawing diagrams of the Enterprise, showing where the berthing compartments are, how the engines work, what the chain of command is, everything right down to what's on the crew's galley menu, then arguing online with other Trekkies about their creation of "facts" that don't even exist.
In many ways, DU is like a Trekkie convention. They operate within the same framework of mythology, but it's a "big tent" with lots of competing ideas about who is the best Captain, the evilest enemy, the fastest ship, etc. All utterly disconnected from reality, of course, but harmless fun nonetheless.
Harmless fun, until you show up at work one day with your foam rubber Spock ears on and start speaking in Klingon. Then someone throws a net over you.