I myself like to watch to see if a bug I find in my garden is a pest or a benefit because I can use beneficial bugs to help me in the garden. Some flies can aid in controlling other insect pests (hover fly larva kill aphids). Flies also are good pollinators! Only bees and a few wasp pollinate more plants than flies.
They eat many kinds of decomposing organic matter, including algae, carrion, compost heaps, manures (rabbit and chicken), mold, plant refuse, and the waste products of beehives like it's candy! They not only eat all that but they also eat meats! That can come in handy if your trying to get rid of bodies! LoL!! Just kidding:)