some will have, say, 5 stages to reach climax, then 2 dong strokes to fill 1 stage or 3 pussy strokes to fill same 1 stage. Thus stroking dong makes her cum faster. Some mood can have 20 stages, but 1 dong stroke against 5 pussy one, some it is much faster to stroke dong.
Purpose of it will become obvious when I add fatigue (which is there, but restored automatically right now). Girl have limited amount of fatigue and you won't be able to make her cum forever, at some point you'll have to either wait some time or go shopping to restore her fatigue and start "production" again. Player gets currency by celling cum, so there is a game loop :)