The Compress commands enable you to deflate and inflate files. When you want to make the file smaller you use the Compress.deflateFile method. When you want to return the file to its original size you use the Compress.inflateFile method.
Deflating files is extremely useful for transferring files across networks and the Internet. If your application/screensaver downloads files from a server, you could deflate them before placing them on the server. This means your application is downloading smaller files, which saves on bandwidth and, consequently, money and time to download. Once downloaded your application/screensaver would inflate them for use.
This also makes sense for other distribution methods, such as USB keys, and CDs for very large applications/screensavers.