I have acquired quite a number of these over the years, although I still don't know a lot about them. Carrier cancels are nearly always back stamped, and most often are found on covers from the 1870s and '80s. They are not scarce if you keep an eye out, but don't show up all that frequently. Most often they are from large metropolitan cities, which had the resources to hire carriers. I have not seen one from any small towns.
As I understand it, most mail at that time was held at the post office for pick up, but some mail was deemed important enough to be delivered by carrier. I'm not clear on whether the carrier applied the back-stamp or if that occurred at the PO before the mail was handed off. If anyone has more information about these I would love to know more. I've never understood how mail was determined worthy of carrier delivery; there was no additional fee.