Shell accounts were useful and thus popular 20-30 years ago when UNIX systems were still something relatively exotic. It was a cheap way for the average home user to get limited access to a computing resource that was otherwise completely inaccessible to them. (Indeed, my first exposure to UNIX was via an SDF shell account, after trying in vain for hours to get RedHat to work on my parents’ old 486.)
Today, Linux installs easily on pretty much any modern consumer hardware. VPSs are a dime a dozen (and much more powerful and cheaper than what SDF offers). Is SDF’s user base purely driven by nostalgia (they still offer dialup access for $7-10/month [0]), or does it provide some useful service not available anywhere else that I’m missing?