The biggest benefit of using thick paint is that it is easier and faster to build opaque color as opposed to areas of color that are more transparent. Acrylic paint, by its very nature is quite thin. It often takes three coats to make a color solid and be uninfluenced by colors or values beneath. Painting as thickly as possible can reduce the necessary number of coats to one or two. This simply saves time.
Sometimes we want transparent paint, and sometimes we want opaque paint. It is helpful when starting a painting to use the paint more thickly so you can more clearly see the colors and values. A color or area will be more like itself if it is not influenced or changed by what is beneath it. Using the paint with less water allows this to happen.