I try to make my team better by assigning points to the right quality for every player, by changing strategy overall and have individual settings for strategy for every player. I even pay huge bonusses to my players and make sure they're happy.

But I lose a lot from players that keep bringing new players on the fiekd during a game. Why is this and how does this help winning? It can't be that they're tired, because they sometimes start as early as the first minute. From another topic, I understand that it's not some bot playing.

Could someone update/teach me why this is and what the result is?

I may be wrong but I think it is to give more players training points from the matches. I do it for that reason when I am connected during a game at least.

All they are doing is putting on substitutes