Why are some fats used in varnishes and paints?

For painting, varnishes are considered to be the best, to which fats and slowly drying oils, or organic resins are added. But in the case of fats, they must be persistent, otherwise in the process the latter will disrupt the normal course of the drying process of the oil contained in oil paints; Dried oil paint contains fat and oil in its layer, and fat occupies a significant part, which has an excellent effect on the quality of the picture and the durability of the painting. Also, the grease prevents the paint from cracking when it dries.