The Dice Roller GUI is a tool which makes it easy to perform dice rolls in Roll20 without needed to type in complicated formulas or roll commands. It's great for beginners who just want to start playing without learning about slash commands or macros.
The Dice Roller GUI does not contain every option available in the Roll20 dice engine, nor is it intended to. It's a way for players to get up and running quickly, but once you have the hang of how Roll20 works, you'll probably want to move on to using the roll commands. Consult the Dice Rolling Reference for more information.