The Designer allows you to store custom JavaScript that can be used globally for views. An example of when this might be useful is when you have commonly used constants or JavaScript functions you want to reference often. You can place the constant or function in the Global Script Resource and reference it, eliminating the need to type it out every time you want to use it. In addition, you could create custom namespaces for your logic.
To access the Global Script Resource, in the Template Designer ribbon, click Global Script Resource. A tab for the Global Script Resource will open and you enter your script in this tab.