There is no single "advantage". Some people just like it, I ran Slackware 13.37 in my laptop for some time: didn't really like it, but the community is nice and I like the "feel" of the distro, the package management wasn't that bad either (using SBOPkg).

Some people might say it's old and unnecesary, I say there are still people who love this distro, and maybe you will love it if you try it.