Now that I'm an apartment dweller, I don't have much use for one. Used to own one when I owned a house, and they do have a multitude of uses, as stated elsewhere.

Now....

Works like a charm for cleaning burned eggs off the frying pan...

This is one of the most versatile tools available. One can use an angle grinder to cut metal, aggresively sand metal or wood, cut tile and stone, and with wire brush attachements remove rust and scale. I probably use it most for sharpening stuff, even shovel blades.