As Michael Chion puts it, "when we dive into our memories, we are diving not into our past, but our present".
The voice from Krapp's 'past', is almost painfully dislocated from the man we see on the stage, shambolic and frail, though it speaks for him more plainly than his 'real' voice ever does. His fiddling and frustrating attempts to get the correct tape from his archive and into the machine beautifully stage our faltering and fragmented relationship with our memories and the fruitlessness of trying to get those same memories to speak or live for us in the now.
The sparse room in which the 'action' takes place is almost an antechamber or waiting room for forgetfulness, in which Krapp's number is never called.