Web Usage is meant to give visibility on the web traffic crossing the router from the LAN into Internet.
It is disabled by default, and perhaps it should stay that way unless you have some legit concern e.g. looking after a minor, need to create statistics, etc.
Clicking Enable » will take you to Administration/Logging page
Monitor Web Usage: On/Off
Monitor: Select the Device/s you are wanting to monitor (All, The following, All except)
Number of Entries to remember: Domains/Searches will limit the size of the logfile. To remove the limit set this to 0.
Daily Backup: If selected will
Clear Data After Backup: if selected will empty the logfile after the backup is performed
Backup Directory: specify where the backup files are to be stored. NOTE the content of the default \tmp will not survive a reboot. You might want to consider USB/CIFS/JFFS as an alternative.
NOTE: If the end client you want to monitor is running a direct TOR/VPN connectivity into Internet, Web Usage will not work.