It seems that it would otherwise silently skip the rescan for pre-backup used keys that are still waiting to be generated into the keypool?

See also #11249 
CHECK_NONFATAL(key.VerifyPubKey(pubkey));
        CKeyID vchAddress = pubkey.GetID();
        {
            pwallet->MarkDirty();
            
            
            
            for (const auto& dest : GetAllDestinationsForKey(pubkey)) {
                if (!request.params[1].isNull() || !pwallet->FindAddressBookEntry(dest)) {
                    pwallet->SetAddressBook(dest, strLabel, "receive");
                }
            }
            
            if (!pwallet->ImportPrivKeys({{vchAddress, key}}, 1)) {
                throw JSONRPCError(RPC_WALLET_ERROR, "Error adding key to wallet");
            }
            
            if (pubkey.IsCompressed()) {
                pwallet->ImportScripts({GetScriptForDestination(WitnessV0KeyHash(vchAddress))}, 0);
            }
        }
    }
    if (fRescan) {
        RescanWallet(*pwallet, reserver);
    }
    return UniValue::VNULL;
},
    };
}
RPCHelpMan importaddress()