karate shop: power stage?

[add4]

Hello all,
i'm would like to try to put a karate shop being a TS type overdrive (i love doing this with 2 pedals) in order to give that resonant filter option in a pedal.
however, i don't get what is happening in the power stage of the karate shop: what is the function of this network of resistors (R15,R16,R17, of C12, C13 , D1 and the transistor?). the diode has to be for polarity protection, but the rest.. why can't i use the traditional caps for stabilizing the 9v and then the voltage divider + caps again for have 4.5v for biasing theopamps?

thanks