Why are harmonics important to Rife machine effects?

Rife used sine waves in his original experiments. Despite the fact that sine waves consist of a pure frequency and contain no harmonics, Rife's original machines still produced harmonics because he used plasma tubes (see below). Because any non-sine wave contains harmonics (i.e. frequencies other than the fundamental) it's possible that the bioactive effect associated with any given frequency may actually be due to one of the harmonics and not due to the fundamental. Because of this we tend to use complex waves on the principle that even if we are not using the correct fundamental frequency to kill or devitalise any specific pathogen then we are probably hitting a harmonic that does.

A plasma tube is a complex non-linear electronic device. What this means is that it tends to distort anything you feed into it in complex ways. So even if you fed a pure sine wave into a plasma tube you would get harmonics because the wave will be distorted by the tube.