Science

Oscillators for computers are quite easy to detect because they create an enormous series of harmonics, and relatively strong ones. This is because a typical oscillator for this application is square wave, and an ideal square wave is the sum of sine waves starting at the fundamental frequency, with each 3rd harmonic 1/3rd of the power as the previous (so if you have a 25MHz oscillator, you'll have the fundamental frequency of 25MHz at amplitude 1, 75MHz at amplitude 0.33, 125MHz at amplitude 0.11 and so on ad infinitum). Eventually you may end up with a PCB trace or wire that just accidentally happens to be the right length to be resonant, and that harmonic will radiate quite strongly.

When your intended signal is minuscule, it doesn't take much to have it swamped by some unintentional radiator - at the very least it's extra noise you need to deal with.

A good example of unintentional radiators swamping a signal, a problem with some aircraft VHF com radios when receiving is that something like the 11th harmonic of the local oscillator (used to tune the radio) on certain VHF channels will completely swamp the GPS L1 band as it leaks out the VHF radio. The signal strength of this harmonic is absolutely minuscule, but it's huge compared to the GPS L1 signal, and it's the LO in the receiver rather than a spurious emission from the transmitter (part of the procedure of fitting a new VHF radio to an aircraft is making sure it doesn't interfere with a GPS receiver, and if it does, a stub filter usually needs to be fitted to the antenna port of the radio).