Accessing the discount window allows institutions to vary credit conditions (i.e., the amount of money they have to loan out), thereby affecting the money supply.
If the lending banks are unwilling to offer enough credit at this rate, the central bank may step in and make loans itself through the discount window.
Liquidity provided through open market operations is not efficiently distributed among banks in the interbank market and there is a case for discount window lending.
Some other means are: discount window lending (as lender of last resort); moral suasion (cajoling the behavior of certain market players); and open mouth operations (publicly asserting future monetary policy).