Voltage regulator using bipolar NPN transistor
What is the reasoning behind using the NPN-transistor?


The NPN is configured as an emitter follower and basically this means if you set a certain voltage on the base (using the low power zener), the voltage on the emitter will be between 0.5V and 1V lower on the emitter across a large range of load resistances.

In other words it's a unity gain voltage amplifier - whatever current is demanded by the load (within reason), the output voltage will be largely maintained at a fairly constant value. It's not as good as (say) an integrated linear regulator like the LM78xx series but it has its uses.


  What part of the circuit draws the most current?


With no load connected the zener draws the most current. With a full load on the output, the NPN takes the most current in order to feed that current to the load.