A square wave is a compound of sine waves, namely the basic wave plus all the harmonics – the same as pressing one key on a piano will generate one sound that is characteristic for the piano because it can be resolved into a number of sine wave frequencies that resound all at the same time when one key is pressed. In this manner, it is not necessary to focus on a single or on a set of sine wave frequencies.
In that a square wave is in reality a compound of many sine waves, it is like using a shotgun as it issues many frequencies at the same time. Instead using sine waves from a frequency generator is more like shooting with a rifle, you work with specific frequencies. This is a more targeted way of zapping.