Routing a PCB is like constantly flipping your ruler around: Old, classic through-hole parts are in inches/mills, while new, modern surface-mount parts are in millimeters. All electrical aspects (traces, spacing, lengths) are measured in inches/mills, and often the mechanical aspects are measured in millimeters (if the enclosure is not made in the U.S.). And if you use the wrong system, you engineering tables and formula stops working, the whole thing is out-of-context (e.g. measuring the gap in a differential pair in millimeters, the number makes no sense), and an integer becomes a float with three significant digits after the decimal point.

So here is the joke: The electronics industry is moving towards the metric system, one mill at a time. At least CAD programs have a button to switch between the two instantly.