These blanks should be filled in using the words "inflation" and "economy."
The main job of the Federal Reserve is to manipulate the money supply of the United States. The point of this is to allow the economy to grow at a rate that is not too fast and not too slow. If the economy grows too slowly, the Fed should increase the money supply. If the economy grows too fast, we can get excessive inflation. In that case, the Fed should reduce the money supply to keep inflation low.
The Fed, then, manages the money supply in an effort to ensure low inflation and steady economic growth.
I agree that the main job of the Federal Reserve is to manipulate the money supply. However, it is not a benign manipulation. The Fed was established in 1913 after secret meetings (read Creature From Jekyll Island). The Fed was sold to the American people that it would reduce or eliminate inflation (inflation was predominantly stagnant in the US since its inception) and create stability in the market. It did and was created to do the opposite.
Since the Fed was established, the value of the dollar has consistently gone down and is estimated as being worth 5 cents on the dollar. Also, most volitility in the market prior to 1913 was a direct result of the same immoral bankers who started the Fed. Run on banks happened prior to 1913 because either their practices were self serving and they lost trust with their investors or they were malicious acts initiated by competing banks.
The core reason for inflation in the US is from printing money out of thin air as our Federal Reserve system does. By increasing the amount of currency in circulation, the value of each note decreases. It is as simple as that. Any complexity is merely a means to keep you from seeing the wizard behind the curtain.
So, my answer to the question posed is: The Federal Reserve manages the nation's money supply to keep the elite in power of this country through their debt creating monstrosity!