Updatable queries allow you to create pages with quite complex business logic. For example, you can insert or update records in multiple tables simultaneously transparently for a user, who just fills an Insert/Edit form and presses Submit. To make a query updatable, it is necessary to provide INSERT, UPDATE, and DELETE statements that are executed when a record inserted, updated, and deleted accordingly. Please refer to the manual to learn how to write such statements.
You can find another example of using an updatable query in our NBA Database Demo. Starting from version 18.3 the same technique can be also used for views.