If only a VoIP phone is attached to the port, then PQ for VoIP traffic would offer very little benefit, but it won't hurt. Because of the latter, i.e. it won't hurt, often it's enabled.
For a trunk, or an edge port that has, for example, a VoIP phone and PC on port, PQ can be critical for VoIP traffic.
Again, since there's usually no downside to having PQ for VoIP traffic on any port, often it's applied to all ports to make maintaining the network a bit easier (as then you don't need to worry about when and where it should be applied - if fact, having a "standard" [logical] QoS policy, to be used everywhere, makes network maintenance easier).