Watch your flies...

What are flies (the insects) for? Most creatures have a purpose, like bees pollinating flowers, do flies do that too? Or is their purpose just to be annoying to horses and people? I've just been outside cutting the grass and there were 2 or 3 flies buzzing around my ears and trying to land on my nose.... consistently. You'd think that they would get fed up being swatted away, but they don't learn, just keep on trying.

Our house is the first place that I have lived that has fly-screens; and also place that seems to have the most flys (or is that flies, I can'd decide). In the 5 seconds is takes me to open the fly-screen to get back into the house, a fly managed to fly in with me, then decide to fly out again, then in the last nanosecond fly back in to the last inch of the door opening. Does anyone have any tips for a fly-free home?