I can't stress enough how fun this league is. I've completely given up playing single player because it doesn't live up to competing against actual humans - and the people in this league know their stuff.
To echo the comments above, there totally is an entire fictional universe that the member's maintain.
There's even a book to prove it 
And while the baseball stuff is great, the members themselves are genuinely great people. We've had gaming groups (Civilization, Diablo) and we've even had several
Mafia/Werewolf games run right on our forums.