Today I got a picture of this fly pollinating the cilantro plants. He carefully went from flower to flower, not even trying to get into the house.
The reason creatures pollinate plants is interesting though. I can think of four off the top of my head. The first one, being obvious of course, is nectar. They go for a quick bite to eat and pollen gets on them for their meal ticket.
A not so obvious one is they think they are going for a different meal like the case with the corpse flower. It smells like rotting meat and attracts all kinds of things.
Another form of trickery is found in an orchid. It mimics a wasp on it's flower luring in other wasps to mate with it. Instead it's of course a flower and gets pollen for it's trouble. Not at all what was promised but this wasp is a little gullible and does it again.
The fourth one that comes to mind will strike you as obvious, people. No trickery here though. People often pollinate plants for a variety of reasons, food, flowers, or just reproduction like trees they can sell.
So next time you see a fly and want to squish it, think about the plants it helps pollinate and think if it's really a good idea or not.