UI scripts are quite a lot like Script Includes. While Script Includes are libraries of code that are available to any script running server side, UI scripts are libraries of code which are available to any script which runs client side, on a page where the UI script is included (or if the UI script has the Global tick-box checked, then code within it can be called from any form, list, or page on the client.

UI scripts are a great way to avoid re-writing the same code over and over, and maintain a central repository for a given set of methods or APIs. This can save you time, and help with maintainability.

Of course, having code available on the client means that it must be sent from the server to the client, and be loaded in the user's browser. This can have performance implications during page-loads, and whenever these scripts are invoked. For this reason, it is wise to be judicious about using the Global tick-box on UI scripts. Instead, it would be wise to specifically load these scripts on pages and tables where you know they'll be used, and avoid loading them when they're not called or don't serve a purpose.