"I am still learning" ~ Michelangelo’s motto.
It allows for a variety of colours to be used without making a dominant feature, and by substituting detail with variation in colour and/or tone you create interest in a subtle way.
It creates soft edges. Hard edges (wet-on-dry) draw the eye, remember, so having soft, wet-into-wet edges creates contrast. By using the wet-into-wet technique in the background, aerial perspective is created.
This technique also creates nebulous passages of colour that help set mood and atmosphere.
One cannot predict what will happen, it's a matter of reacting to, and using to your advantage, what happens on the paper. It is the joy of watercolour! For all of these reasons, to me, it is the essence of watercolour painting. The wet-into-wet technique involves "watching paint dry", but in this case it can be far from boring, but you have to be fearless and brave with it!
I think it is the hardest of the watercolour techniques to master so I made the following notes which, although complicated, I hope will help...