I’ll find it easiest to explain why datum points are usually used with mechanical components. Take a cam shaft, with several cam lobes on the same shaft. With the camshaft’s axis horizontal, there are then a series of “offset cylinders” that butt up against each other(so it’s a chain of rectangles with some poking out above, and some below) It usually makes machining easier .because the machine can be zeroed at one well defined/understood point, and no calculations needed during machining…. There’s also something that I see as being somewhat akinto the Heisenberg uncertainty principle. This is you can control the length of a shaft, or the length of the components, but not both If you measure from a datum point, then you have firm control of the length of the shaft (to the accuracy of the measurements), equal control over the positions of the edges of cam flanks, but the width of the cam, is of double the error. If instead you specify the width of the cam, you can get the width with m