Many repo counterparties that provided liquidity in the pre-2008 era were not banks and thus did not hold bank reserves. Today, with many of those liquidity-providing counterparties unable or unwilling to provide liquidity, liquidity comes from the banks via the Fed. When a bank transacts with the Fed, the Fed either adds or withdraws reserves to the banking system. Thus, when assessing the level of outstanding liquidity, a glance at banking system reserves provides a good gauge. Think of reserves as a footprint of Fed actions.