Emphasis on the memory management: because early Macs had only 128k/512k of RAM and a single floppy drive, dialogs and code fragments had to be constantly swapped in from application disks; and because early Macs had no paging, resources were accessed via handles.

You'd often have enough of a word processor resident in memory, to be able to work with a document disk in your Mac. But if you wanted to (say) print or run a spell-check, you simply didn't have enough memory to do so: so the System needed to purge resources, and load the requisite resources (code, dialogs) from the application disk. You'd be constantly swapping between user disks and application disks. Resources and handles were the way the System constantly shifted parts of an application in and out of the limited memory, tracking where they came from.