Because the brush is damp, some of the water in it will go onto the canvas along with the paint, making it thinner than it is straight from the tube. One of the secrets of successful glazing is thin paint (find out what the other secret is here).
Knowing whether a particular color is transparent or opaque is also important. For instance, the quinacridone gold being used on the brush here is wonderfully transparent, but another of my favorite colors, titanium buff, is extremely opaque and needs to be thinned considerably more. In fact, I always use acrylic glazing medium with it, not just water (see this Acrylic Painting FAQ for more info on this).
