(You can do other stuff with suitelets) But for the most part, they let you display data to the user in a more "custom" manner. By default though, they don't have any functionality built in when you add a submit button and click it. So, you'd attach a client script to the suitelet that says "I pushed submit" what should I do? which you specify when you create a button. So when the button is clicked and it calls the method, that method lives in the client script that you attached.