Every transaction -- deposits, withdrawals, transfers between accounts -- costs your bank for providing the service. Most banks have, at minimum, computers that record and process every transaction, and other transactions require one or more employees, such as bank tellers or merchants. Chances are the bank will allow you an unlimited number of free transactions with your checking account. For savings and money market accounts, though, the bank might charge you a fee.
Even if all of your transactions are free, though, federal regulations require that banks limit you to only six transfers per statement period from your savings or money market accounts [source: e-CFR]. If you've already made six withdrawals from your savings account in a one-month period, the bank is obligated to stop you from taking out any more. This can affect your overdraft protection service and, possibly, cost you more in fees. Be sure you know how your bank handles overdrafts if it can't transfer money from your savings account.