Banks must exchange funds and negotiable instruments constantly. You deposit a check drawn on 'Bank A' to your checking account at 'Bank B.' 'Bank B' must present that check to 'Bank A' and obtain funds. But 'Bank A' also has checks, some of which are drawn on 'Bank B,' so the funds transferred between banks must be the net of checks drawn. That's not a particularly difficult process when the banks are "correspondent banks" but gets complicated when there's NOT a direct exchange relationship between the two banks. That's where the "clearinghouse" function comes into play. The TOP "clearinghouse" is the Federal Reserve Bank. Larger banks serve as "clearinghouse banks" at lower levels. It would be impossible for every bank to have an exchange relationship with every other bank ... and it's similarly impossible for every two banks to have a common clearinghuose bank. That's where the Fed comes in.
When a bank, after processing all the checks it can drawn upon its own accounts and those drawn upon the accounts of correspondent banks. bundles the remaining check and delivers them (before midnight) to its local Federal Reserve Bank, it then must wait a specified number of days,
as established by the Federal Reserve Bank, for the Fed to post funds to that bank's account at the Fed. That's called "Float" - and it's the amount of time that a bank (including the Fed) has the USE OF FUNDS before it must credit those funds to a depositor's account. Historically, the Federal Reserve has varied this period of time to manage the Money Supply ... which "heats up" or "cools down" the economy by controlling the rate at which cash is cycled through the system.
There are remote/rural banks that have few direct relationships with other banks and with a more distant relationship with their nearest Federal Reserve branch. Corporations often choose such banks for their Accounts Payable accounts and obtain beneficial terms for their business. The reason has to do with the amount of actual (real) time it takes for chekcs drawn on such accounts to actually clear the system. This is one of the ways in which corporations take advantage of a game that banks have played forever.
It's been many years since I worked in the banking industry ... but I doubt this fundamental has changed much. It's the Holy Grail of commercial banking.
You can read up on the ACH (Automated Clearing House) system to get a better understanding.
http://en.wikipedia.org/wiki/Automated_clearing_house