What am I missing?

Assume there’s your favorite booster at the input, what is D2 doing? This is the most common clipping arrangement that I have seen. It seems that D2 does nothing.
If the boost at the input is running off 9V then the you should have a single that is centered on 4.5V going past the two diodes. D1 clips the signal when it is above 0.7V. D2 on the other hand should not allow a signal to pass until it gets above the reverse bias voltage.

Feel free to let me know what I missing if anyone has some insight into this.