Stuffit Expander is a free decompression utility that may still be of use to some users -- especially, for example, when trying to decompress older Stuffit (.sit or .sitz) archives.
Many users will do fine with just the built-in Archive Utility in Mac OS, which handles .zip files and a wide variety of .tar archives and tarballs -- but older and boutique utilities like Stuffit Expander can help when handling more unusual formats, such as .rar files and esoteric suffixes like .kgb, .7z, and .arc. Stuffit Expander lets you decompress files with a quick drag and drop, or through the file menu, and the app handles more than 30 file formats.
If you often need to decompress unusual and/or legacy files, Stuffit Expander can be a good addition to your toolbox. If you're looking for compression capabilities or more-advanced decompression options, check out Stuffit and Stuffit Deluxe.