That's not always the case, but for those where it is, it makes some sense. It only takes one pistil for seeds to be fertilized, but it doesn't guarantee that the pollen will find it way to the pistil. In insect pollinated plants, the insects may visit a different species next, so the pollen would go to the "wrong" type of plant. And the wind can carry the pollen in the "wrong" direction. So by producing more stamens (and therefore more pollen) it increases the chances that at least some pollen gets to the "right" place.