Need To Understand Transistor Based Led Circuit
First, let's be clear that this is a crappy circuit.  Five LEDs in parallel is not a good idea for one thing.

However, the point seems to be that Q1 will be off when the input voltage is "low".  That allows Q2 to be on, which switches in additional parallel LED load resistance.  For low input voltages, the resistance in series with the LEDs is (R24+R25+R34+R35)//(R8+R9+R22+R23).  For high input voltages, Q1 is on so Q2 is off and the LED load resistance is just (R24+R25+R34+R35).

You can think of this circuit as being a two-range current regulator.

Overall it looks quite wasteful and more complicated than it needs to be.  It woul help to know the two zener voltages and acceptable range of input voltage.