For the unique static identifier linked to a permanent record on a distributed ledger that you own.

It's a safety net. If your nsec is compromised you would want a way to rotate the keys that is not accessible simply by way of having the nsec in hand. If all you require is nostr events, then the bad actor that grabbed your keys could post these same events in your name and permanently lock you out.

It is also service-agnostic and nostr is not the only reason you might use this, just currently the most obvious one.