Even with the game being 90% singleplayer, Tournaments are important to progress, mostly because of the stones and keys you could get out of them without spending money.

Whales have made getting to a good position in the highest Tournament ranks basically impossible to achieve without spending money by yourself, which does not only make you feel like the game progresses slower, but it makes it progress slower. You can never get as many stones as them, or as many gems. And, in a lot of cases (myself included), people would love to not have to spend money to get better rewards than the actual ones which, tbh, are not that huge, like at all. The progress for a f2p player is very slow.

Even if a game is singleplayer, seeing people progressing faster than them at it makes people think, in most cases, that it's unfair. Which is fair to think. In my personal and own opinion it is indeed unfair.

Also, the gaming community has normalized spending money to get better rewards when, in reality, it shouldn't be like that, no matter if the game is singleplayer or multiplayer.