If you have Activity Reporting turned on, the computer will collect information you can use to monitor the activity of the user account. Perhaps there are inappropriate sites or applications you did not think to block. Maybe the user account is spending more time playing games than you would prefer. By using the Activity Reports, you can monitor the usage and ensure that your Parental Controls settings are effectively protecting your children as you intended.
Windows Vista Parental Controls lets you set a frequency to be reminded to view the Activity Reports and Vista will alert you when the time comes.