More players on the bench?

Because you'll have more choice. And many players - especially the young ones - don't have much space during a regular season. You have to loan them to make them grow, and you can't even control how they play if you loan them abroad.

If you can bring 12 players on the bench and the result of the match is positive for your team, you could make a substitution with a player that doesn't usually play much. They can grow up faster and, especially, like I said above, you'll have more choice during a match and not only the same 5 subs with the same players for every game.

So yes, they'll have more total time for playing during the season (if you like to play in a realistic way like I do of course).

This brings more reality. The rule of football says the bench size is increased since 10 or more years now. Like they update the rules of the leagues, or other stuff like that, they should improve this one too.

Anyway the reason why we need more subs is the same reason that made the IFAB decided to change the rule in the real world football. Why in Serie A they have 12 subs and not 7? The answer is the same for the videogame.