The main idea is to have a simple way to write code while developing without breaking anything. It can also help to debug elements and have developer shortcuts to test gameplay.
I always like to develop with little planning, this can cause all sort of problems and mess up with the code. One way to get over it is defining a global variable "mydebug" that you will be set while developing.
If set then a group called "Debug" is activated and any code that you place in there will run. This is code that the final player will not see, code that allows you to "cheat" on the game or just code that you are testing before you know it works.
Any code you develop and works fine can then be moved outside the Debug group to your main "clean code". If you want to deliver a playable version, you only need to unset the "mydebug" variable and only the "clean code" will run.
See below the link to the manual reference for Groups in event sheets.
Event groups manual entry