Gambler...
you pick your opponent, wager, time, game, rules, spot if any and table or in other words.. you play at just about your comfort level or just at the point that makes you bare down.
Gambler in Tournament...
you pay an entry fee....
and then the TD tells you:
your opponent, time, game and format, rules (and enforces them) and if any the TD tells you the spot and then what table to play on, and in some cases... what time you have to be ready to play.
If you gamble a lot, playing in a tournament can be giving up a lot of control of your game and what you want to accomplish.
Why play your speed when there might be someone in the tourament that you can 'control'... and match up later?