When you want to make a more musical synth, it is hard to design an analog synth from the beginning.
This chip contains a VCO inside, and SAW, Triangle, square wave is output.
What the user does is just add a voltage 0〜10V according to the frequency.
Oscillator sync is also possible by adding this chips or a master clock.
Since it contains only basic elements, it is easy to expand afterwards and you can create a synthesizer that contains the elements you want yourself.