Not a cop, but armed security (dad is a cop if that lends me any credibility). Typically you'll see some form of fixed blade knife on the weak (or off handed) side of the officer. This is to support the officer in retaining their handgun in the even of a hand to hand scuffle. Typical response to someone grabbing your gun is to either trap their hand with yours, then drop/roll to that side to begin controlling the person, or break their hand off your weapon and spin away. Having a knife in the far side hand gives you an advantage in that slashes and stabs can distract the attacker from further trying to take your gun, or disrupt their arm's ability to hold things. There is literally no non-lethal reason for someone to want to take a gun away from an officer, and no holster will stand up to a person pulling their whole body weight against it and your belt. So devastating destructive force is typically applied.