Those purchases put downward pressure on longer-term interest rates generally and helped normalize the spread between mortgage rates and long-term Treasury rates, which had widened during the financial crisis. Reducing longer-term rates influences the economy in much the same way as lowering the expected path of short-term rates.
For instance, the decline in longer-term rates lowers the cost and increases the availability of capital and credit, which in turn encourages business expansion. In the most recent episode, another important result of lower rates has been a reduction in debt service burdens from existing debt. Households in particular have significantly reduced mortgage payments through refinancing.