Sine waves are simple things and provide a nice entry point to learning audio signal processing. So far we've been able to discuss things like frequency and amplitude using nothing but individual sine wave oscillators.
But the thing is, a solitary sine wave is not very interesting to listen to. It's a blank slate, devoid of any color, character, or drama.
Things start to get much more interesting when we have multiple sine wave oscillators at our disposal. What we can do is combine them to produce more complex sound waves. This technique is called additive synthesis.