Depends on the length of print runs. Data spooled to the printer without the HDD is stored on your computers hard drive and fed out to the printers ram as the ram empties data to print, using system resources. An installed HDD allows very large amounts of data to be sent directly to the printer and stored there as it prints, system resources are freed up once the data has been sent. You can also store large quantities of of fonts on the printer, access is by use of http://h20331.www2.hp.com/hpsub/cache/3 ... n/gc121306
There is IMHO no requirement for an installed HDD for short print runs and limited fonts. These are usually deployed in situations where multiple users need sophisticated printing options.
Edit:- This worth a read. http://h20000.www2.hp.com/bizsupport/Te ... D=bpl03728