This game is about managing probabilities. You can manage probabilities with all your decisions. Win bonuses will probably slightly increase some of your players attributes, and as a consequence you have more chances to win the game. In fact, it’s like real football, if you pay a win bonus to your team you should have more chances, but the effect is impossible to quantify ir to explain rationally... Nobody likes to lose when having a better team in perfect shape and motivation, but if you win all the matches where you are better than your opponent, the game would be very boring...