Explanation of capacitor usage in this N-MOS application
I can see an application for this.
If you put a 12V clock  on buzz so this is attenuated to 4.5V.
Then DAC can be a sine or triangle + voltage that modulates the buzz smoothly so it sounds like this on a speaker.
Let it capture a few bursts then press play.  You may change the clock to any f and modulation triangle as you see fit.