I've got an old Macbook Air that has to be used with a USB sound device because the internal hardware died at about 4 years but wasn't worth replacing the motherboard. In other cases, I'm sure professional (or even hobbyist) audio people use hardware other than the computer's built-in sound devices. How long do the makers of those keep updating the drivers when it's a 5 generation old product and their developers have moved on to newer projects? A driver they gave you for OS X 10.5 isn't going to do you any good today.