One reason I can think of is if you do a lot or repair/refinish work, you find out not all cues are .850. I see the range from .830 to .865. I have a friend making some sleeves now for .860 and .865. I then make a brass mandrel to press them on. Low cost but effective for when you need that odd size. I have a set from Tom at .850 and I use that for my cues that I build but I also have .830, .835, .840, and .845 that my buddy turned for me out of carbide sleeves from drill presses. It is too expensive to have a complete set made for every size at $250- $300 a pop, just on the off chance you need that size to refinish the shaft of a customer's cue.
Dave