if you check this box, the variables become global meaning they will be available directly on the page.
For example, if your custom code is myVar=1; and the box is unchecked, this variable can't be read when you refer to it either directly from the page or later from other Launch components — execute myVar in the browser console and you will see "the variable is not defined".
With the box checked, you can read and write to it directly from the page — execute myVar in the browser console and you will see "1" returned.