Why did Fed buy MBS?

Can you elaborate on the reasons behind the Federal Reserve's decision to purchase Mortgage-Backed Securities (MBS)? Was it a proactive measure to stimulate the economy, or a reactive response to specific market conditions? What impact did these purchases have on the housing market, the broader financial system, and the average American household? Did they contribute to the eventual recovery, or were there unintended consequences? Understanding the Fed's motivations and the outcomes of their actions is crucial for assessing the effectiveness of monetary policy and preparing for future challenges.