Starting a painting with a layer of Liquid White is part of the Bob Ross Wet-on-Wet Technique®. It's neither better nor worse than any other painting technique, and has its place in art. Myself, I really love the richness you get with a good, well done underpainting, but that takes more time.
The point of the medium-covered canvas is that the painting develops quickly. It is a wet-on-wet technique reminiscent of watercolor paintings. You "wet" the canvas with a covering of either clear medium (Liquid Clear or Magic Clear depending on brands) or white medium (again, Liquid White or Magic White) Then you work the initial layers of pigment (paint) into the medium. Because of the medium, the paint goes on very fast, very loose, and you need very little. The idea is to finish a painting in a two- to three-hour session. So it lends itself for class instruction, plein-air painting or such.
The point of the Liquid White is that it lightens and mutes the pigments as they are applied. It does make for really beautiful skies: when you apply the pigments, full strength to the the upper and outer corners, then pull in, the blue of the sky naturally lightens as you reach for the horizon. A lot of artist have trouble with this, and for a sky to really look real, you need this gradation.
The same is true when you start applying distant trees, or mountains or whatever. Because of the layer of white, any light color is again a bit lighter. Then subsequent layers can be added, slightly darker and more detailed as it comes to you. If you are going for the smoky-mountain effect so prevalent where I live, it makes an ideal way to start a landscape.
Under the right circumstances it is a good method. You will not get the same results you will with doing an underpainting, but you can get some great results. I have done a lot a paintings that start this way. I do not hesitate, however, to add to such a painting later, when the first layers have firmed up, and dry brush in details.
You can tint the canvas before, even lightly sketch in some major blocks. I started both of my lily pad paintings this way, with the Monet version tinted pink first, with a layer of white to soften the colors of the pond.
