Winrar is far less widespread and the compression algorithm used by RAR archives is proprietary, whereas the ZIP file format and compression algorithm are well-known and patent free, and have been made to work just about anywhere (any reasonable Linux distro can open ZIP files out of the box, Windows has had built-in support for ZIP files since XP). So long as you're comparing just these two programs, the fact that ZIP files are almost guaranteed to work anywhere is in most cases good enough reason to only use ZIP archives, in which cases winrar is just winzip with some unnecessary functionality piled on top.
Still, I'm pretty sure people buy winzip out of habit or ignorance. The kind of people who used winzip when there were no well-known free alternatives, and who don't care to look around; it works for them, it's convenient sticking with something they know, so they're happy. I doubt you'll find that sort on a computer forum, though.
Oh, and of course, do what everyone else here is doing. Use 7-zip. It's faster, better and also free. Join the club. We're all super cool and popular, and none of us got where we are now by using winrar.