On a mountain or touring bike, a triple will give you up to a 5.5:1 total gear range with stock cranksets, and wider is possible. With a well selected cassette they can do this with a nice double shift, that's wider in the granny range (1.25:1 per shift) and narrower in the upper range (1.15:1). You spend most of your time in the middle ring, with the big ring for descents and the small ring for bailout.

You probably don't need that range on a racing road bike. For an all-rounder, it's handy.