Mana concentrates in bodily fluids. This is the logic for sex as a method of healing someone or restoring their energy - a rather flimsy and obvious method of selling a game as an eroge, but nonetheless that's how my world was designed and those are the rules of magic here.

Supposing that someone was in no shape to have sex, or that I was in no shape to provide any at the time, having some stored away for later use would be helpful.