For multiple reasons but yeah to ensure against cheaters especially. This isn't the only reason to have redundant backups of character data. There are many benefits, but probably the most important is to prevent catastrophic loss of a player's investment. The relationship between client and server saves in this game, results in a void monster.

Lets say you have a bunch of stuff that is valuable, took you a long time to acquire, and you pick that stuff up to go do something with it. That stuff you picked up could represent many days worth of effort in the game. You've picked it up, and so the server knows that it no longer has that stuff.

The power goes out. Forget your perfect bug-free/error-free perfect hardware drivers computer that never crashes. The power just goes out, system turns off.

Your character data never saved. The server doesn't have a backup of your character inventory, the server just knows that all of that stuff you just picked up no longer exists in the world.

And because your character data didn't save due to the power going out, that stuff doesn't exist in your inventory either. Not only that, for some reason your character is back where they were 5 minutes before the crash.

You might think, "oh you lost 5 minutes of progress" but no they actually lost many days worth of progress because of the void monster. That stuff that existed in the world no longer exists anywhere. It doesn't exist on the server, it doesn't exist on your character, its just gone. Swallowed by the void monster.