Don Stewart, from ArtistChronical (now defunct), walks us through the basic steps of toning a canvas in preparation for oil painting. Why would you do this? White is very, very, bright and so that causes you to view the colours in your work much differently than if you use a neutral tone for the background. Basically, the brightness of the white changes the apparent contrast and can make colours look much darker as a result.
Don touches on some of this in his video:
Interestingly, it's not common to tone paper for watercolour as this is primarily a function of watercolour relying on the white of the paper for the whites in the image. Having said that, for scenes in which white does not play a role, the use of a toned watercolour paper could have interesting results. Joseph Turner was known for using toned paper for landscape scenes, choosing the colour of the tone based on the scene he was painting.
In any case, give the concept a try in whatever medium you're working in, you might find you like it.