driving mosfet gate with an npn bjt
For a penny or two more you can drive the MOSFET with a push-pull circuit. 







simulate this circuit &ndash; Schematic created using CircuitLab

In this circuit Q1 and R1 level-shift (and invert) the input to drive Q2 and Q3 which are connected as complementary emitter-followers. Peak turn on current is limited by Q3 hFE and R1, so if R1 is (say) 4.7K, the current is in the hundreds of mA. 

As far as the circuit you've got, if you have a relatively high load current, however, the 150 ohms may be considered justifiable. If the MOSFET is spending most of its time 'off', however, it's perhaps wasting significant power. Keep in mind the engineer who designed it was probably more interested in keeping their job than making the circuit a little bit better- the cost of those parts might pay for his or her salary. 

The transistors are called upon to do two things- to drive the relatively huge gate charge of the MOSFET and perhaps to level-shift the input if it's less than Vdd. If the dimmer output was coming directly from a chip such as LM555 runing at 12V then there would be no need of any drive circuit.