To protect the base-emitter (BE) junction from significant reverse bias. For NPN transistor, positive base potential relative to emitter cause the junction (which is like a diode) to be forward biased and start conducting. With forward bias, transistor works as a current controlled device. More current between BE means more current between CE. But BE junction may be damaged if reverse bias (base negative relative to emitter) is applied. BE junction of a typical small transistor starts to breakdown at around 5-7V of reverse bias voltage. This breakdown may damage the BE junction, reduce the hfe (basically gain) or cause total failure. Because in this circuit input signal from antenna is unpredictable, the diode is used to clamp any excess negative voltage up to safe limits, so base does not go too much negative in respect to emitter.