Do you mean why a UPS beeps when it's disconnected from power? I imagine so the user knows it's disconnected.

Annoying during a power outage though.

I assume they meant why is the power interrupted if it switches from charging to not, and I believe the answer is "it's cutting from just vampire tapping the incoming power to feed outgoing, and it would need either chunky capacitors or to be constantly wearing on the battery to not do that".

You might be able to hush the beep by holding down the UPS ON button.

Works on all APC UPSes that I've tried, at least.