Physical techniques are integrated with firearm techniques to allow you to deal with immediate, in your face, aggression where either time or distance do not allow you to fully present your weapon. They are also used when the decision to fire has not been made [ie an unknown person is suddenly in your space but does not appear to be posing a lethal threat, but must be controlled,moved etc to allow yo to deal with a known threat]. They are also used to more effectively deal with multiple aggressors. Additionally, they can be utilized to foil the attempt by the aggressor to bring a weapon into play [ie close range confrontation, your movement has been impeded and the aggressor is attempting to draw a weapon].
Realize that most crimes against individuals occur at very close ranges, generally inside of 5 feet. When reacting at that range, the advantages to closing the gap, instead of trying to create distance, are tremendous. Of course, if your only mental framework for defensive skills is shooting at 7yds (the mythic 21 feet), you will be way behind the power curve when having to respond to a situation at 5 feet.
Wheelman,
The main problem with practicing off-hand shooting and gun-hand striking is that most people carry their gun on the side of their body that they shoot with (right hand shooter, right side carry), so it is impractical to assume that you will be able to get to a gun carried for right hand/right side access into a firing grip with your left hand while performing some sort of gross motor (striking, covering or intercepting) action with your right hand in the face of close range interpersonal aggression.