This may be useful when sending jobs to service bureaus that are still in OS 9.
Also, while most current font manager applications can "bridge the gap" between the OS X native and Classic environments by making fonts you've activated available to both environments, they seem to be limited when it comes to data fork-based fonts. For example, your font manager can't make your .dfont fonts available to Classic because Classic doesn't understand how to use them. dfontifier can help solve this problem by allowing you to convert your .dfonts into regular resource fork-based TT fonts. That way, you can be certain you're using the exact same font set both natively and in the Classic environment.