The lav is locked so we (pilots) don’t have to wait for the bathroom to become available. Purpose is to reduce the time where only one pilot is at the controls on international flights. We try our best to sequence the bathroom breaks all at once during crew rest swaps to limit the inconvenience to the passengers.

We put the napkin there so we don’t accidentally open a locked lavatory with someone in it.

Hope this helps explain why we do what we do...