Ever since stereo records came out in the 1960s people have been trying to get their mixes to sound as wide as possible. People think that having all their tracks recorded in stereo means that their mixes will be guaranteed to be wide, but that's usually the opposite of the way things turn out. If you're guitars are panned hard left and hard right, and so are your pianos and drums and everything else, then there's no contrast in your mix. Everything is in both speakers and the ear has trouble separating your mix.

Contrast can come from many different avenues, and taking mono sources and panning them around is one of the easiest and best ways of creating contrast in a mix. If you are given source files that are all stereo, make sure they're truly stereo, and not just a stereo track. If the source isn't truly stereo, consider throwing away one side and panning the remainder to help it sit in the mix or stick out of the mix. You could also use any number of plugins to reduce the stereo width of a track. Most plugins that aim to widen the stereo field can also reduce the stereo field, so don't think of them as one-trick ponies. Confining the stereo width of an instrument can help you place it in a specific area of the stereo spectrum.