You really only
need StuffIt if you want to download files that have been compressed with that program. They'll mostly be older files.
Originally, before Mac OS X, compression programs weren't shipped with the OS. So they shipped StuffIt Expander with it. Then, once Mac OS X was released, the Unix utilities like
tar,
zip, and
gzip had troubles with certain kinds of files: those with resource forks. Eventually, Apple gradually moved away from supplying StuffIt and enhanced the Unix compression utilities to better handle Mac files. So they stopped including StuffIt Expander.
The $50 version of StuffIt probably has useful enhancements for making, browsing, or extracting archives. It used to, anyway, but I haven't used it for a few years now.
charlie