Because watercolour dries so fast, and because it is very difficult to alter when you have made a mark or finished a wash, watercolour painting lends itself to working quickly and retaining a fresh quality to the pictures.
I enjoy the way colours can be allowed to flood into each other in just one wash, and then transparent washes can add another layer of interest.
Combined with some pencil outlines or with pen and ink, watercolour also makes a great sketching medium, ideal for working outdoors.
It is also ideal for certain kinds of illustration work as it dries one day and you can put it in the post the next day. Unlike oil paint!