2, reasonable distribution of machining allowance.
3. Provide the basis for positioning, clamping and alignment for subsequent processing and inspection.
The final size of machining cannot be directly determined according to the scribing, because the accuracy of the scribing size and position is not high, and the line itself has a certain width, so it cannot be accurately interpreted by visual inspection. (another theory that doesn't require precision at all)
My views are for reference only.