Welcome. Stuffit is a company that releases the program Stuffit Expander (free) and Stuffit Deluxe (not free) to open .sit and .sitx archives.
If you want to make those archives, you need Stuffit Deluxe, which you pay for. The problem is the compression is proprietary. There are plenty of alternatives (like zip) that are free to open (and natively in Mac OS X). They are better alternatives because a) they are built in and require no outside program, and b) they are free compression algorithms.
Companies like Stuffit like to get as many people to use their .sit and .sitx archives as possible because it keeps them in business and takes money from your pocket.
Stick to free alternatives so you don't hand over control to companies like Stuffit who can take their compression in any direction they feel like. There is nothing stopping them from charging for Stuffit Expander. Now if everything on your computer was compressed with .sit, then you wouldn't be to happy about that...
They just added rar support (and others) to get more people to use it.