You're doing the wrong comparison. It's not about if one thing matters more than the other. Bonuses it's an "addition" to your team's performance, so obviously the more bonuses you have, the better you team will perform.

It's like watching the match or not. You get a +8% in possession. Of course you can win without watching it, but why not to have an extra +8%? Same with training bonuses, +10% is better than +6%, and you will have more chances of winning with +8% than with +2% (seems obvious to me).

That doesn't mean you can't win without them, like you can win without watching a match. Just your chances will be lower.