Why do we...?

We use acronyms all the time: PIN, ATM, VIN. And all too often we add the word that the last letter of the acronym stands for after the acronym. How often have you heard "PIN number" or "ATM machine" or a "CDL license?" What is actually being said is Personal Identification Number number, or Automatic Teller Machine machine, or Commercial Driver's License license. Isn't that a bit redundant?