Re: I'm no rocket scientist
IIRC India didn't have a reliable, powerful liquid engine for the first stage, but their ICBM programme had given them plenty of experience of building big solid engines, so that made sense for the first stage - which is the third largest solid motor ever fired after the Shuttle SRB and the Ariane V boosters. The second needs to be less powerful and more controllable, so they could use their moderately powerful liquid Vikas engine which is derived from the original Ariane 1 motor.
I'm not sure why they then stick another solid on top of that. But they do need a more controllable liquid engine to put satellites into orbit.
India has now mastered big liquid engines and cryogenic engines for the GSLV Mark 2 and Mark 3 respectively, so I'd imagine the PSLV solid-liquid-solid-liquid sandwich won't be repeated.
I bet it makes an amazing noise though - one day I'll see a rocket launch. The Reg should open a travel tentacle for us space nerds.