When you have a jumble of units, it is hard to relate them. If an acre of land gets an inch of rain, how many gallons of water is that? If you did this in millimetres, metres, and cubic metres, it is easy.

If your car's motor is rated in horsepower but electricity is measured in kilowatts, how do you compare those?

What about fitting copies of a product measured in ounces in a shipping container measured in short tons?

Typesetting measured in points, but paper measured in inches?

Fitting metalwork done in mils but woodwork done in binary-fraction inches in a house measured in feet?

The list goes on and on. It's easy to say "but my domain is special and deserves its own units". It's much harder to resist and say no, there is a perfectly suitable unit with power-of-1000 prefixes to use in your application.