Watercolors are an amazing medium, in part because of the fluidity of the paint itself. The inherent characteristics of water (surface tension, etc) allow artists to do many things with watercolors that other mediums do not.
For example, in the video below Judy Morris demonstrates how to paint a perfectly smooth, incredibly even wash of color across her paper. She lets gravity do most of the work, and merely “guides” the color where she wants it to go. Take a look: