It is a mixture of different opponents and the fact that people that normally people that play in tourneys are a little more serious about the game and will potentially provide better competition.
You also have the environment that is conducive to contemplating the game without many of the distractions you might have in casual play or by playing online.
I care more about the outcome of each game when it's in a tournament; in particular I tend to dismiss losses in casual games, but I revisit losses in tournament games and try to figure out what I did wrong. I therefore learn more from my tournament games.
In many of my casual games I will break off calculation and play a move on intuition where I would continue to calculate in a serious tournament game. My record for tournament calculation is 28 minutes on an endgame manuver (I'm proud to say that I got it right). Even if the time control allowed it I'd seldom put that kind of hard work into a casual game.
The exception for me is Online Chess with multiple days per move: those games sometimes get a decent amount of attention from me. But I use the analysis board and openings databases, so they are not ideal practice for standard play.
OTB tournament play is light years ahead of online play.
1. Your opponent will actually shake hands and say Good Luck.
2. After the game your opponent will say Good Game.
3. You can ask your opponent if they want to go over the game, and majority of the time they will.
4. You get to make new friends.
5. You get to see old friends.
6. You get to hang out with fellow players that come up together from the same club.
7. You get to experience to places.
8. You get to hang out at Mels til the early morning doing analysis.
9. You get to play pai gow with GM Enrico Sevillano (True Story!)
10. No one accuses you of cheating/Sandbagging.
Online chess:
1. Hardly anyone says Good Game/Good Luck. Online its look upon as insulting???
2. Havent had 1 person offer to go over a game.
3. Accusations of cheating/sandbagging.
4. Get cussed at.
5. Get insulted.
6. Excuses made after a loss.
7. Acting like an idiot after a win.
8. Quit a game while losing.
9. Post insults on your profile page.
10. Getting blocked.
The difference is obvious, and illustrates how powerful the annonymity of the internet is.
Usually the only way to get better and better at chess is to learn from your losses and maybe games where you thought you had the better position but drew.
Tournament play is set up so that eventually you're going to get plenty of disappointments. Can't duck players who are better, maybe a lot better. The disappointments are not just over the board but related to the respect of the good players who you may have to play and see often face-to-face.
This combination usually separates the players who won't accept not being good from those who don't care or just drop out.
On the plus side, if the better players recognize that you really want to improve. At tournament's, after a game, they will usually discuss key points and that info is invaluable.
There's a great YouTube video of Kramnik analysing with a pretty young Carlsen at I think Corus in 2007 maybe. It looks like Carlsen thought he had chances in what I believe was a drawn game, obviously a player that was intent on always improving.
Many people say that playing tournaments is more helpful than simply playing without going to tournaments.
What is the main difference that makes tournaments a better way to improve?
Is it simply different opponents or something else?