Many good reasons have been said.
I see another one, happens to me from time to time; The entertainment of the own developer.
Maybe you have all the story set and all the characters set (they can be little or many, depends on the game). Also they can be many just because they're "npcs" that are just there to make the world more alive and not all about trying to fuck your main goal/s.
One day the dev is making some conversations with the npc and suddenly decides that want to make the MC fuck her, because she's hot, of just because why not. He may make that scenes and let them in or out of the game. (depending on the game it will be a pro or a negative).
The only down take to this, is that if time is on the budget and patreons are waiting to get what they expect. Some will appreciate the addition, some not.
This reason has nothing to do with adding main characters that suck time from other characters. That is just bad planning, but doesn't mind that is a good or bad choice per se, because the new char could be more interesting than the old ones.
(and also, other of many reasons because I don't wanna release on patreon, I want to make the game like I wanted and take my time and release a finished product, no pressures.)
