Apparently, it is supposed to give your players more desire to win games. One would assume that it increases your possession stats or some other stats that increase your chance to win. I would assume 2.5/5/7.5/10%, but unless I have missed them posted in another thread, they could be anything, or nothing.
At level 3 and 4, I am winning consistantly and with large scores WITHOUT the win bonus.
I used to have it maxed for every game, league and cups included, and realized I want/need the money more.
I still have the CL win bonus but it is set at $80K and I get $240K/group and last night it was $676K, so $80K seems a good deal if it gives ANY better chance at winning.