Because I could fax it to multiple airlines and get credit all across the board. For example, If I was flying one star alliance member, I could fax the boarding pass to every member airline I have an account with and get miles on all of them.

I have been lucky, the one time there was a hassle getting miles (canceled flight and they put me on another airline) United did it all over the telephone with just the ticket number, no boarding pass necessary (though a ticket number was, and it's not on most receipts, so make sure you get an itinerary printed out wither online ar at the airport)