The availability of different caliber barrels was/is a big selling point, and I used single-shots for working up handloads, or shooting sketchy/questionable ammunition.
At one point I had a used Sears-branded single & it totally wouldn't have been anything special.... except it came with 8 barrels in foreign & domestic military calibers, including one odd belted magnum that I never messed with. It came in handy for finding out how old surplus or handloads behaved, before running it through something scarce & expensive.