Boxer receives a punch, then he tries to move backwards along the direction of the punch. This will reduce the impact or the impulse of the punch.
If the boxer remains stationary or moves forward, then the impact of the punch will be much higher, and will harm him.
When he moves in the direction, the duration of contact will be longer during the punch. So same impulse in longer duration will have less force. Also, the relative velocity or acceleration between the two bodies -- punching glove and his body will be reduced.