Simply stated, the perfin is for private use in marking the stamp with small holes so as to discourage employee theft. Stamps can and do still get perfined today, although they are much less common because postage meters have replaced stamps in most large businesses.
The precanceled stamp is that authorized by the post office to be placed on large mailings for use ONLY at the post office printed on the precancel.
I suppose the theory is that if a dishonest employee were to steal precanceled stamps from one business, there would be the opportunity to re-sell them to other businesses using the same post office. If the stamps were perfined, it would prevent that possible re-use.
As you may know, locally precanceled stamps (showing city and state designations typically between two bars) have been discontinued since 2007. Today's precancels are still in use, but are a bit more generic as shown on this USPS web site:
http://pe.usps.com/businessmail101/...canceled.htm