Another reason: stability, predictability and preventing runaway. If you don't use R2, and your transistor starts to heat up driving those 5 amps, Veb wil decrease, so the base current will have to increase to compensate the drop, which means that the collector current will be amplified, which means more temperature, etc. The voltage divider R1/R2 adds some stiffness to Vbe. Using a diode thermally coupled to the transistor, instead of R2, could be a crude temperature compensation trick. Using a couple of diodes instead of R2, and a small emitter resistor would be a classical temperature compensated constant current driver. Since the emitter resistor would disipate a lot of power at 5 amps, subsititing R2 by a whole temperature compensated active load is the best option: I've seen that done in some power amplifiers.
Strictly speaking: if you don't use R2, base current is given by: Vcc = Veb + Ib * R1. This should be solved via load line analysis (or using the Lambert function), since Veb is a logarithmic function of Ib, and a rather complex function of temperature. If you use R2, this resistor will be in parallel with the dynamic resistance of the eb diode: if R2 is significantly smaller than the dynamic resistance, Veb will be more dependent on R2 than Ib. So the circuit will be more stable and predictable.