- Surface Tension
Anything that aids flow reduces the surface tension. This means instead of a drop of liquid creating a "bead", it will be more likely to just flow. When it flows more naturally, everything gets an even coat of paint, unlike a wash where the crevices fill up more.
What do I mean by "surface tension"? When you fill a glass all the way to the top with water, the water kind of curves out a bit instead of spilling immediately - that means it has a high surface tension.
Off-topic example: Vinegar in a small bowel will attract fruit flies, which land on the liquid and drink - surface tension prevents the flies from sinking. A little dish soap mixed in with the vinegar will lower the surface tension and when the fruit fly lands, it can no longer walk on top of the liquid, sinking to the bottom of a bowel, drowning.