When painting on canvas I ALWAYS use portrait or fine grade canvas so I find that I am able to use several different "types" of brushes successfully and am not restricted to the use of bristle brushes to get into the deep grain texture of the lower grade canvases. Using top grade products increases the beauty of your work, causes less damage to your brushes, and provides a painting flow that is so rewarding.
I have said before that the paint you use, your surface, and the effect you want to achieve will dictate the type of brush that you will want ie: soft, semi soft, medium or stiff.
The type of brush ie: flat, filbert, etc. and size is dependent upon the artist and what they find is more comfortable for them to produce the effect they are wanting to achieve. The specialty brushes make painting easier..... An experienced painter might be able to " make" just a couple of brushes to work but why struggle? Why not make painting a truly fun and rewarding experience with the ease of what is available to be used.
I DO NOT feel that brush companies provide several lines, several types of brushes, etc. to promote sales....I feel they provide selections to make us better artists by providing individuality.