I can answer this.....well give a reason why I'd want to but cant since mine is too old to have this feature.
Some sound via set-top boxes and certain channels is incredibly quite and not very clear. If it were not for the appalling mids and bass, at times I'd have the TV do the sound. So for those times I'd like the brighter sound of the TV mixed with the soundbar.
But I think the newer Q80/90 have better decoding so might not have this problem, in which yeah its a strange thing to do.
Sony allow you to use the TV for the dedicated centre channel for mutli-channel too, which is even worse if you ask me.