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 
{
        AssertLockHeld(cs_wallet);
        assert(m_last_block_processed_height >= 0);
        return m_last_block_processed;
    }
    
    void SetLastBlockProcessed(int block_height, uint256 block_hash) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet)
    {
        AssertLockHeld(cs_wallet);
        m_last_block_processed_height = block_height;
        m_last_block_processed = block_hash;
    };
    
    void ConnectScriptPubKeyManNotifiers();
    
    void LoadDescriptorScriptPubKeyMan(uint256 id, WalletDescriptor& desc);
    
    
    
    
    void AddActiveScriptPubKeyMan(uint256 id, OutputType type, bool internal);
    
    
    
    
    void LoadActiveScriptPubKeyMan(uint256 id, OutputType type, bool internal);