The fed purchased long term assets like mortgage backed securities to provide liquidity and relief to corporations that held too much bad debt on their balance

The fed purchased long term assets like mortgage backed securities to provide liquidity and relief to corporations that held too much bad debt on their balance sheets. This provided immediate relief for distressed institutions. The fed also expanded the balance sheet by selling long term bonds, which offered relatively lower rates in comparison to short term notes and bills, this provided the cash infusion needed for QE to stimulate the economy.