When you have your kick drum recorded and you've spent hours trying to squeeze out a fatness that isn't there, maybe you can use artificial tricks to enhance the kick drum sound. Enhancing the low end by adding a sine wave generated by your DAW's oscillator is a great way to add thump and boom to your bass drum track, whether it be rock, hip-hop or whatever genre you choose to make.
A sine wave is the most simple sound wave you can find, representing only the fundamental frequency you choose — in this example, 50 Hz. It doesn't include any harmonics, unlike the square or sawtooth waves. So by adding a sine wave to the kick drum we will only be adding one tone, and not a musical element or characteristic.