Banks and other depository institutions borrow from the Fed to meet reserve requirements. As banks overextend lending, they may be caught short on reserves. Federal funds and the discount window are ways banks may obtain the reserves to meet their requirements. These are both good for short-term deficiencies in reserves. A financial institution is more likely to borrow from the discount window when the discount rate is low, as it is more likely that banks will be easy with their credit, since the cost of borrowing from the Fed isn't too high.
Monetary policy is imprecise.
The Fed depends on the banking system to carry out its policy objectives and must predict rather than dictate bank behavior.