maybe one of you can shed some light on why anti virus software quarantines instead of deleting?

what it does when it does this?
why can it quarantine and not delete the file?

i recently had a virus called jeefo (see other thread inthis forum), NAV could not delted the file as it said it didnt have access to the file. after doing a full system scan, i found 6 infected files which i had the option to quarantine only. NAV took about 20 mins or so to quarantine these 6 files! is that normal?