There is no reward. a reward is something you get FOR doing something. Fun is what you get AS you do something you enjoy....
A reward doesn't need to come after a completion of a task to be a reward. Nor does the game need to specifically incentivize every action to acknowledge them in some manner, or for them to be rewarding.
Fun is the reward of gameplay.
I think there is a translation error here, but I don't agree with you.
There is a word useful - i.e., it gives access to something else. And there is the word funny - which brings nothing further.
If you are implying that ganking has no utility...that's debatable and subjective to one's goals.
I've talked to gankers and they usually kill for fun, not that it's useful to anyone, even them.
Fun is useful to them. The entertainment value alone is going to be the overwhelming incentive for most people to play most any game.