From some tuning guides on racewaxdotcom, it seems like you brush to get dirt and grime out before a hot wax clean cycle, then you hot wax clean a couple times, then you wax for real and brush the wax after you've scraped. So it seems brushing is for pre-wax cleaning and post-wax excess removal and wax texturing.
But then, from some of what I've read in some threads, it seems like the stiffer brushes (steel and brass) are used to actually texture the ptex base material, not just the wax on top. Is that a structure maintenance action to do between stone grinds? or is that actually done as an alternate structuring method? And if you're brushing to actually deform the base material, do you do that every time you wax or just every now and again when you feel the structure needs it?