I bet it's Usenet users. WinRAR can split large files into chunks that are uploaded as separate Usenet messages.
And also have parity built in for file recovery. The alternative will be to use par2 to create parity files.
The parity files are the killer feature for me. Probably 95% of the downloads from Usenet end up needing them.
7z also does this now FWIW
I'd still bet it's Usenet users that installed WinRAR way back when and have stuck to it ever since
That's me. But now everything is done automagically by nzbget and I use nanazip on my Windows desktop.
Better compression over .zip and other older formats (like .gz).
Why people use it over .7z though? For that, I have no idea.
rar is super popular in China, because for a long time (and still with many modern implementations) it is much better at preserving Chinese filenames in Windows than zip.