(1) they're cheaper to add than real button holes, and (2) they're much easier to remove or move if you need to adjust the sleeve length the only downside is that they don't actually work... but since I never actually use them that doesn't matter.
Usually about $5-$15 per hole depending on if it is done by hand or by machine.
One thing that another member wrote about some time ago that I thought was interesting... He mentioned just to have whichever holes you are planning on leaving undone made functional and leave the rest as non-functional to save $. Makes sense provided the non-functional ones can cosmetically look the same.
Another thing to make sure if before having it done (a good tailor will confirm this with you rather than just try to "wing it"), is to ensure there is enough fabric/overlap on the sleeve/cuff/material so that they can accommodate a working cuff without having to add extra material or make it look sloppy.