Stuffit Expander is not only for Mac OS X. It's because many Mac users make use of this expander and decoder utility very frequently. You can easily get Stuffit Expander for free online, as it's very popular. However many sites offer the same link if you are able to download the expander from the site itself.
Stuffit Expander is a utility that compresses files using the GZ compression algorithm (which is used in several other computer programs as well). The main purpose of this algorithm is to compress files to make them smaller in size when they're stored in an archive. So, by compressing the archive, your computer will save much more space, as it will not have to hold so many data. Stuffit Expander is a utility that is not able to decompress, hence Mac OS X will not be able to run certain programs that require decompression. For example, if you want to run a game that is designed for the Macintosh, then Stuffit Expander won't work for it.
However, this doesn't mean that all is bad with the Stuffit Expander software. In fact, the software will help you to make a large number of archives, by allowing you to create a number of different archives according to the file formats required by the programs which you're currently working on. Also, it comes with a support for a number of common file formats, which means that you don't have to convert your archive every time you want to use it. However, it doesn't come with any protection for your information from being stolen, which is why you should use some form of protection, even if you're using Stuffit Expander.