If your team participates in many competitions during the season and you want to win all these competitions then you must have 2 equally good starting elevens to achieve that and if you don't have 2 equally good starting elevens then you must choose what competitions to give priority and what competitions to sacrifice otherwise you are risking to fail in all competitions and here's why...
Every time when your player plays in match he gets his hidden attributes "Jadedness" increased and it can stack. The more matches the players plays in the season the higher "Jadedness" he has at the end of the season.
The major thing about "Jadedness" is that it responsible for how fast your players burn their "Conditions" during the match, for example, near the end of season before the match you see that all your players in the starting eleven have 95% Conditions and the opposition players also have 95% Conditions so you see it and thinks that the opposition doesn't have any advantage above you in that area but it might be wrong because all your players might have very high level of "Jadedness"(which you can't see because it's hidden) about 300-400 and the opposition players might have very low level of "Jadedness" about 50 so after 1st half the condition of your players will drop to 60% and they will be just walking on the pitch and the conditions of the opposition will drop only slight to 85% so they still will be fresh and smash your team in the 2nd half.
So as you can if you team has 22+ players that equally good and your opponents have only 11+ good players then at the end of the season you will be having advantages above your opponents because of it.