Multiple birds with one stone...
> Why does my copier/MFP/workgroup printer need a hard drive?
Spool space, yes, but also for a couple features that practically no one uses:
secure print facilities (send your sensitive job to the printer, it spools and holds it on the drive until you enter a PIN)
frequently printed forms/job templates (Never seen this feature in use in the wild, but I know it's there. I've seen the job templates used before, though)
mailbox capabilities (I.E. scan a document to the printer for a particular user, it'll store it until the user comes up to fetch their stuff- also one of the 'never seen in use, but it's there' gags)
temp space for scan to email functions (dead useful for quick and dirty document capture!)
general spooling if the device has fax capabilities and some plonker forgot to feed the device paper (or if someone is printing over 9000 copies of 'war and peace' and is hogging the print engine's time)
It's also used for doing such things as running multiple copies of BIG jobs- example, I had to make a couple copies of a 300 page manual one fine evening that was double sided, hole punched, and collated- could not have pulled it off it the hard drive in that copier failed.
Keep in mind that hard drives are generally only installed on either workgroup or larger class devices as well- most of the canon Imagerunners and HP MFPs come to mind.