Each table in an app share the same goal. And that is the goal of the app that holds these tables. For example, if you were building an inventory management application, the goal of this app is to help the users keep track of the inventory, its locations, quantities, orders, lead time and so on. Each table in that app helps you further that goal. Now, in this app, why would I create a payroll table? If I did, it would remain typically remain unrelated to the rest of the tables in the inventory app. Hope that helps.