JSModem for JSLinux

Perhaps to start a single-user BBS directly in your web browser? Then you just distribute your IP address to your friends, and they can dial in (one at a time, of course) to chat with the sysop and upload new warez onto your enormous 60-megabyte hard disk.

A lot of JavaScript and HTML5 demos are about replicating 20-year-old computing experiences.

The same kind of thing you'd use JSLinux itself for (ie. nothing, it's just a very cool proof of concept).

Teaching. Usually, if you have a lab doing operating system classes or security. You usually need to setup virtual environment for small activities like showing how a Unix environment behave even if you get rid of half of the binaries. Here, you just need to restart the web browser to restart the Linux to its initial state. Compared to VirtualBox or VMware, you don't really need anything on the client PC beside a recent web browser.