The result is face runout of TIR <.05mm on standard collars with even tighter tolerance on bearing locknuts and special designs.
A precise shaft collar face to bore relationship is important for many applications. It ensures that components are both properly positioned and not tilted on the shaft.
Collar face perpendicularity also ensures even pressure on interfaced components. Uneven pressure can result in premature failure of components such as bearings.
When shaft collars are subject to shock loads, precise facing reduces the likelihood that the position of the collar will be shifted.