A performance exists between people who cannot reproduce that exact meeting tomorrow.

A performance happens between people and not merely in front of them. The particular room, the particular attention, the particular night: these are half of what occurred, which is why the same players cannot produce that evening again even with identical material. What you attended existed once and included you in its making.

This is a reason to be somewhere fully when you are there. Recording it captures the material and not the meeting. And if you missed something and cannot get it back, the honest truth is that it is gone, which sounds like loss but is also what made the thing worth attending in the first place.