I should have written more about the why question:
For susceptibility, you don't want your electronics to get burned out, your munitions to blow up, or your life support systems to fail when you key up a nearby radio transmitter or get illuminated by a radar, whether the emitters are installed in the same vehicle or just nearby. For emissions, you don't want your electronics to jam nearby radios or radars. Everything has to play well together, while your opponents are trying to kill you at the same time.
US military items with EMC requirements are usually required to pass MIL-STD-461 requirements. It's a very tough specification to meet!
One commercial device I had to have tested as a plain old FCC part 15 device had problematic spurs from a roughly 16 MHz clock over the limit line all the way up to about 1 GHz. Those high upper limits such as 18 GHz, 26.5 GHz, and 40 GHz are there because harmonics of digital signals cause emissions problems even well beyond the 50th harmonic of the fundamental.