A great many images and scenes involve passages where detail and texture are best added while the initial wash is still wet. This soft edges of the secondary information keep it integrated into the general statement. This prevents the detail from being too specific and attracting too much attention.
Take a look at the grasses in the foreground of this landscape. The soft edges allow the varied strokes to read as all one thing, keeping the area from becoming too busy. It manages to be complex and simple at the same time.
Here's a similar foreground in which the artist has made a general statement with an overall light green wash. While that was still wet, he added vertical strokes of varied color and value. If the green wash had been dry, the foreground would have overwhelmed the composition (it's a close call even so).
The following photos include some areas where this concept would be useful. Try out adding complexity to a wash that is still wet. The real job here is keeping track of how wet the brush is. After the wash is applied, the brush you used still has enough liquid in it to pick up some more pigment from the palette. Stay out of any puddles there, and don't stick your brush into the water bucket. If the paint on the brush seems too dry or thick, remember you are about to add water to it when it touches the wet paper.
Practice this on a scrap of good paper until you see the results you want. Then you're ready to make a proper painting.