Re: Reminds me...
It's not even that they're designed to run at 80% (or whatever) of capacity, in a lot of cases. Different materials require tools to run at different speeds. Different tool heads require different power/speed combos, etc.
Just for example, running a drill too fast for the material generates excess heat, which makes the drill and workpiece expand, the drill seizes and/or welds itself to the workpiece, and things break spectacularly because the workpiece is firmly fixed to the bed of the machine, which is now trying to twist itself into a pretzel.