It checks for errors.  Silly mistakes that a system admin might make.  Basically, it checks to make sure the system is consistent.  In particular

• The home directory exists.
• The account does not share a home directory with anyone else.
• The account owner also owns the files in their home direcory.
• The shell is on an approved list.
• There has been file activity recently.
• The disk space used by the account is reasonable.
• The numeric user ID is not root's.
• If there is a shadow file, the account is also listed there.
• All mail spool files have an associated user.
• All entries in /etc/shadow also appear in /etc/passwd.