This is not much of a hot topic of concern these days, however when you think back to the launch of Mac OS X and the transition strategy, you might remember the dual-boot PPC Macs, or the later Macs that shipped with the Classic environment/emulator.
It's kind of hard to understand today why Classic was even needed, but you can slowly come to a realization about how just much software there was for the Classic MacOS, and just how much software would be forever obsoleted, by browsing MacintoshGarden.
I'm still on page 73 of the games section, and it amazes me how much of this stuff never made it over to Mac OS X. Granted a lot of this stuff is not particularly high value, but there were games for Classic MacOS in a way that there just never really were for Mac OS X.
I now kind of feel like Mac OS X never had nearly as big of a software library as the Classic MacOS did, which is indeed a strange feeling.