Further points:
- By grouping players into leagues, it makes adding players to your tournament easy. When you press "Add Players", just select the league, click "check all", press OK.
- Leagues make it easy to see statistics on only a certain group of players.
Of course, if you only have 1 set of players (which is probably true of most people), you really don't need to create a league. Then again, you may want to create a league to group your "regulars", and omit players that show up one-time, friends of other players, etc.
- Seasons are really for grouping tournaments, and have very little use beyond the Stats tab. You can filter tournaments to see stats for a particular period of time by setting the Start and End dates in the filter, but it's easier and more intuitive to filter on a season.