Function of this diode in this circuit
Presumably the amplifier can drive its output below ground or the diode would not be necessary. The 2N3055 has a Vbe breakdown rating of -7V so we need to be sure not to exceed that voltage. Reverse breakdown of the base is not good for the transistor- it can cause permanent beta degradation over time, and the 2N3055 is already miserably low gain. 

The diode as shown will conduct when the voltage goes more than about 0.7V below ground, clamping the voltage the transistor base sees to that level (the rest of the voltage appears across the resistor).