It's just a framing device and easy way to justify narration from Bob Saget.
All The League talk in this thread is making me interested in it
It's great, don't really need to know much of anything about fantasy football, any fantasy sports can apply to those jokes and they're maybe 5% at tops and often just the basic premise that sets up other stuff. (Like using trade demands to affect things in real life, etc.)
The first season starts a bit slow but it figures itself and the characters out in the second half of it, then after that it's gold. The same husband and wife team write and direct pretty much every episode so it's quite consistent.
It might actually be the raunchiest (couldn't think of a word that doesn't make me sound like my parents) sitcom on TV. (Not counting HBO/Showtime/etc.) Certainly more than anything else I can think of off the top of my head. Hell, probably even is more so than Curb.