The BC107 is to provide enough current to drive the base of the larger BFY50 transistor. The problem with regular BJT trasnsistors is that they have a very low input resistance. The output current depends exponentially on the base emmiter voltage (a bit like the current through a diode depends exponentially on the voltage across it), so the base emmiter voltage only needs to be tiny to cause a large current in the collector (the output). But the base current depends directly (by a factor of beta or Hfe) on the colelctor current. Many large power transistors have a beta value that is quite small, so when the collector is taking the full rated current, the base current may still be qutie large! This is a problem for the circuit that needs to drive the transistor. If it is say, a small integrated circuit, it wont be able to provide enough current with out exploding to drive the large power transistor. So we need to use a smaller transistor, with a large current gain (the beta) to take the signal from the drive circuit, boost up the current to a level that can drive the large transistor. The is probably why this circuit has a small transistor driving the larger one.