In the very early days, when the Voice Recorder was still being talked about, there was considerable aircrew objection and concern that, even after a successful flight, the possibility was available to listen to the last half hour of flight (as it was then).
So they were given an erase button to remove the possibility.
As the original machine was a tape recorder the button just provided power to a big coil giving a bulk magnetic erase.
I think that these days it is no longer relevant, who cares, but it remains.
If memory serves inhibited until on the ground and parking brake set.