People like their applications to work. Even better, they like them to work, even when things change. For the WinTel world, this isn’t a big deal (Vista aside ;), because the underlying CPU architecture hasn’t really changed, from a program’s point of view, in the past two decades. Unless you have a weird program that’s designed for AMD’s 3DNow! instruction set and you switch to an Intel CPU, or perhaps an application designed for a more esoteric old SIMD architecture, your application should run just fine (as long as your Operating System is ok with it).
Mac OS X doesn’t have the luxury of working on the same underlying CPU though, so things need to be handled a little bit differently. The solution Apple came up with was