A plant that already reproduces poorly on its own has no margin for error. The cacao tree, which breeds badly by nature, depends on a single kind of fly for pollination. Remove that one partner and its reproduction does not just dip, it collapses.
About half of the known fly families are thought to be pollinators, so flies carry pollen for crops like mango, chilli, black pepper, carrot, fennel and onion. Chocolate depends on them completely: a tiny midge is the only pollinator of the cacao tree. Some flowers even trap flies inside and hold them there until they are coated in pollen, then let them go.
Many flies live inside or on other animals and eat them from within. One kind slips an egg into an ant, and the larva eats the ant's head while the ant is still walking around. Another grows inside spiders. A furry, gentle-looking fly that seems to be visiting flowers is in fact laying eggs whose larvae eat the young of solitary bees and wasps.
Because so many flies prey on or parasitise other creatures, they can be used against pests in biological control. Their killing tools are specialised, like a hardened mouthpart that stabs prey through the eye, and some predatory flies can even catch hummingbirds in the air. Their appetite for dead tissue is also useful to medicine: live maggots can still be used to clean gangrene from wounds.
The flying insect we notice is often only the last day or so of a fly's life. Most of the life cycle is spent as a larva that does nothing but eat, and the larva looks so unlike the adult that it barely seems the same animal. Some adults even tear off their own wings partway through life.