Purpose What are flies for?

Here’s what I found:

Cockroaches are potential vectors of diseases such as dysentery, gastroenteritis, typhoid and poliomyelitis. Their diet is omnivorous and includes fermenting substances, soiled septic dressings, hair, leather, parchment, wallpaper, faeces and food for human consumption. The latter may be contaminated either by the mechanical transfer of causative agents of disease from the insect’s body, or by transmission in the faeces. An outbreak of food poisoning in a Brussels hospital subsided immediately an infestation of B. germanica was controlled.

http://www.the-piedpiper.co.uk/th2.htm

Flies: second only to the bees and wasps, flies serve the useful function of pollination of flowers with nectar. [15] Though the thought may be infrequent, some flies are useful: (1) tachinid flies parasitize (prey on) the harmful gypsy moth, and (2) the fruit fly (Drosophilia melanogaster) is widely used in experiments concerning the functioning of genes and chromosomes. [16] Professor Thomas H. Morgan of Columbia University pioneered the subject of heredity in the field of genetics using the tiny fruit fly (Drosophilia). Ease of handling, low expense, as well as, requiring only ten days to complete a generation, made the fruit fly especially suitable for scientists to observe several lifetimes of heredity. [17]

http://fohn.net/pictures-of-flies/significance-importance.html