Why It Matters
The Federal Reserve sets the bank rate, and by doing so it influences the rate at which banks also borrow from each other (the federal funds rate). So if the bank rate is lower than the federal funds rate, banks will probably prefer to borrow from the Federal Reserve when they need loans. This puts downward pressure on the federal funds rate.
Conversely, if the bank rate is higher than the federal funds rate, banks will probably borrow from each other rather than from the Federal Reserve. This puts upward pressure on the federal funds rate. In either case, the Federal Reserve can trigger a change in the federal funds rate by changing the discount rate. This is why the bank rate and the federal funds rate are generally closely correlated.
Because the increase in the supply of funds available for lending puts downward pressure on interest rates, changes in the bank rate can have widespread economic effects. Manipulation of the federal funds rate is one of three primary methods the Federal Reserve uses to control the money supply (the other two involve changing reserve requirements and buying or selling U.S. Treasuries on the open market).
Source: Investing Answers