In theory, a person can just go to a retail store and buy a generic brush that should function well for any conventional art project. This is not always so. Sometimes a brush made from a natural material has an effect and a quality that is highly desirable to more fine and flimsy brushes. The artisan produced and artisan quality brushes such as Kolinsky sable brushes have stiffer bristles that are ideal to applying watercolor-like stains to ceramics in particular.
A brush can have a huge impact on how a medium is applied. This is true for thick paint, but it is also true of fine pigments such as those that are finely diluted. The end result is not easily smooth and homogeneous but has gaps and clear lines where the bristles passed. Depending on the artwork and the technique, this can look great. It is clearly produced by hand and does not resemble something mass produced.