Why is accuracy so important in a CAD drawing?

Because the data is typically used to create programs that are used in CNC programming. As a machinist, I take data from the CAD drawing, the lines and arcs, and use them to produce a finished product that has tolerances in the thousandths or tenths of thousandths of an inch. If a machine tool doesn't get good data, it can't produce parts to the accuracy required for manufacturing or embly.