Yes, to expand a .sit compressed file on a Mac running Mac OS X 10.4 or later, you need to get the Stufflt Expander.
Just download it from its official website, then right-click the .sit file and select Open with Stufflt Expander.
(By default, macOS can only expand compressed files in ZIP format.)
Some background about Stufflt Expander:
The software can handle a variety of file formats, including its proprietary .sit and .sitx formats, as well as other common archive formats like .zip. Over time, as built-in archive utilities became standard on most operating systems and other third-party tools gained popularity, the prominence of StuffIt Expander waned. However, it remains a useful tool for those needing to open older .sit or .sitx files.