The love dart is not a penial stylet (in other words, it is not an accessory organ for sperm transfer). The exchange of sperm between both of the two land snails is a completely separate part of the mating progression. Nevertheless, recent research shows that use of the dart can strongly favor the reproductive outcome for the snail that is able to lodge a dart in its partner. This is because mucus on the dart contains an allomone (pheromone-like) compound that promotes sperm preservation mechanisms in the female.
Huh, TIL that not all slugs use love darts
https://en.wikipedia.org/wiki/Love_dart