For me a big roster is good
- (Maybe) can prevent a bad form for the players who have too many consecutive games.
- Same for the injuries.
- Helps a lot when you ‘re playing in 2 or 3 competitions (as I do) for the bigger part of the season.
Red/yellow cards of course is another reason.

-If you power train a player, better not use him in the games during training period (possible bad performance or injury).
If a player is in bad form (and some time is more than one) , it’s easier to bench him.

-Even if you have the reds to heal an injured player, he must stay out of the games 1-2 more days because it’s possible to get injured again. (Injuries for me is the most important issue of t11, specially lately).
Also you can save some reds from injuries of two-three days.
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Some time, you have two games with 3 hours difference. So, if you have many players, you can save some greens and have fresh players for the second game.

-Maybe someone of the players you bought for your subs, could be the MVP of the season (out of nowhere) .

There is no need to buy expensive players for rotation. Just some 5* veterans.
Avoid to go for bidding war for them (so don’t go for players with sa) and try to find older players 28-30 y.o. with Q_3 or Q_4 (not that a Q68 player is much better than a Q65, but it’s easier to sell him in the end of the season or whenever you want).

The only reason not to have a full squad of 21 positions (I keep one for free agents or some special occasion) is when you have some low q players for “easier draw” purpose or if you haven’t enough money for the wages (mostly for higher level managers).