There are some user-visible changes from rev to rev, like Spotlight (the desktop-grep thing) and Time Machine (versioned filesystem and backup) and fast user switching and iChat and so on. But there are also a ton of developer-visible changes; developers will rely on newer revisions because they have features that make writing programs a lot easier and/or allow writing much better programs; users then have to move to a newer rev in order to run programs which require them. And, of course, plenty of people don't pay for it. You can more or less get by buying every other OSX upgrade. I hear there are some people still running OS 9.