Triggers are implemented at the database layer and fired off by the database engine itself. This means you do not need to modify the code within your front-end application that the users see.
This is particularly important and beneficial when the program code is not in your control anyhow, like a third-party application.
It is also of value when you have multiple interfaces to your data - perhaps via a web app, a fat-client app and a mobile app - whether you can maintain the source code or not. In this case, the trigger can be created within the database and applies no matter how the user is working.
There are, of course, some catches.
I'll describe these on page two, and then give working T-SQL trigger code on page three to illustrate.