Third case (and most important IMO): schema migrations. When doing major shuffling of a schema, you can use triggers to "redirect" accesses to the old tables or

Third case (and most important IMO): schema migrations. When doing major shuffling of a schema, you can use triggers to "redirect" accesses to the old tables or columns into new ones. This way, the schema updates can be applied without bringing down the application. (Such triggers of course are removed once the application is updated, along with any old schema bits.)