Not about not knowing the song, its about remembering how you pre-recorded the track being used/heard out front. By singing along, muscle memory makes it almost seamless in not 'looking' mimed.

In many a video I have made, directors always insist you sing along live and as you would for real, you don't make natural faces or emotions by just moving the mouth like a puppet.

The guide is to lock in as much as possible...performing live is full of distractions as it is, imagine how many are on a KISS stage!