Add-ons create sql tables they need for their usage. When they are uninstalled however, there can remain orphan sql tables. Instead all database access should filter through the XF engine. The addon would invoke a function like xen_create_table("tablename"). This would allow XF to keep track of all add-on created tables, and when uninstalled, to clean up those tables automatically. In the ideal world, uninstalling 100 add-ons would leave the database in the same pristine state as if they were never installed. The primary benefit of this feature is that if the add-on is unneeded, or presents a security risk, its data can be gotten rid of without technical hassle.