If you'd like to see a very exhaustive list of port assignments, IANA is the place to go. This list is the most exhaustive that I've ever seen. This can be useful for things such as establishing 'port forwarding' in a router, which will allow certain ports (such as 548, for Appletalk over TCP) to be sent through your router to one target machine (your Mac running OS 9 file sharing over TCP/IP).
If you are a "power user" in the Mac OS 9 world, and you want to delve into all the services that are offered with OS X, some knowledge of port numbers may be helpful as you start experimenting with Apache, SSH, FTP, Samba, etc. For the typical OS 9 convert, though, you won't have to know anything about them -- other than they're out there, and they help make some of the cool stuff in the new OS possible.

