10 microns = .01 mm = .0004 inch
Tolerances depend on the function of the part and are selected to balance various tradeoffs in production costs and assembly. A well engineered design does not require tight tolerances for the vast majority of features (reducing scrap, tooling, and labor costs), but some specific mechanism components like gears and driveshafts demand very tight tolerances for profile and runout in order to function reliably.
Tolerances will often influence which type of machine tool is used to produce a feature. A tight tolerances on an outside diameter might make the difference between a part being made on a lathe in one/two operations, or requiring additional operations on a cyllendrical grinder. Overzealous requirements for surface finish will require slower feedrates, sharper tools (which wear more quickly) and extend cycle times significantly, or require extensive manual hand-finishing.
