What is the lecture mainly about? According to the professor, why did fifteenth-century painters used impasto techniques?

Listen to part of a lecture in a studio art class.
    Professor:
    Today we’re going to continue with oil painting, but in different styles you haven’t experienced. From now on, we’re going to learn about several different techniques, beginning with impasto. Basically, impasto is a thick application of pigment that makes no attempt to look smooth. Instead, brush and palette knife marks are visible on the finished painting.
    Now, in general, when you work with oil, what you do is just apply one color over another and let the paint squish onto the canvas. And the painting surface remains quite flat. But, with impasto, mostly it involves loading up your brush or painter’s knife with more paint than you’d normally need. Then the three-dimensional paint appears to be coming out from the canvas. It’s sort of like… icing that covers a cake. Actually, when I saw a series of paintings done this way, I thought they were very convincing. They looked good enough to eat.
    Anyway, one purpose of impastoed paint is to make a light reflection. Since the 15th century, impasto has initially been used. Artists controlled the play of light, creating a lot of visual space. The shadows underneath the paint showed folds in clothes and jewels their subjects wore. Impasto really made these features stand out.
    But later, another effect of impasto was its ability to convey movement in the painting. And when you work with the technique, you should keep this in mind that the thicker the paint is, the more it gives physical movement. Let me hold Van Gogh up as an example.
    Van Gogh, one of the post-impressionists during the 19th century, first used impasto for its expressive qualities. Look up at the screen here, and notice how the cypress trees and valleys are depicted by the thick texture. In this way, Van Gogh gave weight to the movement to his sky and landscape. You can almost feel the breeze on that day. Today’s painters use impasto for a different reason. Most current artists place more emphasis on the paintings’ surface, the texture on their art work than the display of colors and lines. Impasto allows them to blend the texture and feeling of an object without illustrating an actual perception of what they represent.
    So, how can you create this texture? Well, it depends on the way you apply the paint on canvas. As I said earlier, instead of "dying" or "scrubbing" the canvas with small amounts of color, just let the paint squish onto the canvas and let it set there. I mean, you apply a massive amount of paint with any tools you find that gives the texture you want. Tools like a brush, a flexible palette knife, or even a toothbrush. After that, you mold or sculpt the paint with short brushstrokes, faster. You should be spontaneous, and dynamic just like Van Gogh.