It's a training tool. Also, I think John had fun writing an interpreter. It's also a very cool language which people without money don't get exposed to. Maybe it gives people some good ideas.
Well, there is always J (jsoftware.com), which is the standard open source APL-like language. It also has a columnar database. And a wealth of resources available on its website/wiki (more than I have seen for any other language's base website).
My C is usually faster than my k: The greatest value in k is that the language is terse and dense; that it promotes the right way to think about solving problems, not that it is fast.
For an intro interpreter/"how do I use K", using Javascript seems like a big win.
If you need to run K in the browser, speed is moot if it's not written in JavaScript.
Is it? I have only seen anecdotal evidence for this.