What does the Fed do that Congress and the treasury couldn't do with out them?
Operate without direct and short term political pressure. Congress did create the Fed.
Our founding fathers recognized that for some functions of government, it is better that they be able to act without the effect of short term poitical pressure of having to be elected, that they can make decisions they think is best for the long term even if not immediately popular with the electorate. Federal judges, for example, are appointed with life time tenure.
This same concept was in mind when Congress created the Fed. With rare wisdom, congress knew that it, a body with direct response to the electorate, could not be independent enough to be in charge of something as potentially devastating as the money supply. So they created a quasi-independent branch that was regularly appointed by the Govt but still had some independence from the political process.
It was very wise of them. We have seen in history the consequences of bad money policy. Many people here blame everything bad in the economy on the Fed. Look at a few examples of hyperinflation to see what really bad monetary policy can achieve.