After some time the database integrity is not ok. This does not always cause problems, but it is better to clean up.
The cause is for example installing and removing modules, which are not de-installing very well.
With the Schema module is it possible to look for the database integrity. The status report has a extra row with database integrity overview.
To have a close look to the database integrity, you can go to menu:
Home » Administer » Site building » Schema
To clean up, you can have a look if all de-activated modules are also uninstalled. Go to menu:
Home » Administer » Site building » Modules » Uninstall
The ‘Extra’ database tables, can be removed with phpMyAdmin.