I always thought field 5 was for comment (although this is typically used for the users real name).
Say for example, you have deleted the user disillusionist
by accident, but his home directory still exists:
ls -l /home
drwx------ 31 1000 1000 4096 2009-05-06 20:55 disillusionist
This would mean that uid 1000 and gid 1000 no longer have valid entries, but the files exist.
Rather than deleting the users files and starting from scratch, you can recreate the group (as root):
/usr/sbin/groupadd -g 1000 disillusionist
Then recreate the user (as root):
/usr/sbin/useradd -u 1000 -g 1000 -c "Restoring an account" -d /home/disillusionist -s /bin/bash disillusionist
Veerloh, you may want to read RUTE