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Why is routing number called ABA/ABN number?
With number of Banks increasing every country at some point in time adopted an Identification code. In US these are called ABA number because they are allocated by American Bankers Association, in UK Sort Codes ... like wise for other countries. See list here http://en.wikipedia.org/wiki/Bank_code In some countries the numbers are given by Central Bank.   To enable internationl payments, the SWIFT body apart from message formats, allocated a  SWIFT BIC [Bank identification Code] so that Banks can be globally identified. Currently  IBAN being adopted in Europe & Australia to identify an Account [at a Bank] Uniquely across globe. In essence these number help uniquely identify a Location/Bank/Branch. The clearing house route the payments or collection instruments to the correct Bank on the basis of this number.