Not being a TD, but knowing some. Reasons for being a TD...
1. Grow the sport
2. Enjoy tournaments and some TDs play in their own tournament
3. Have a sponsorship that 'requires' you to run tournaments
4. Makes money off the side.....selling items (discs, shirts, etc)
5. Runs a raffle where a percent may go to defraying the cost of the tournament (like a 50/50).
I'm sure there are other reasons, but those are some that I have seen or heard about. Number 3 is a big reason for running a tournament. I think (but not positive) that Infinite Discs requires the people it sponsors to run a tournament and Infinite Discs sets up a booth at the tournament.