JavaScript can easily retrieve useless information about the computer screen as well as more useful information about the size of their browser window

Not all of your visitors will use the same screen resolution and even those that do may not all have their browser window maximized all of the time. Even if they did the fixed control bars that some of them will have along one or two edges of the screen will affect the available screen area for the maximized browser window. If that doesn't give enough variations then you also need to consider that different browsers use different amounts of screen space for the browser menus and control bars that your visitors may or may not have visible.

Screen Resolution

If you really need to know what part of your web page is visible in your visitor's browser window (perhaps because you want to display a dynamic effect) then these browser window size and position functions are just what you need.