Offline capabilities + easier to use for some people (one of them being me). Loading up a webpage every time I want to try some new nodejs module would not work as well as opening up a nodejs repl with the module loaded there.
I'm biased of course, and I understand why some people are turned off by an in browser solution. But I think it's probably faster, in most cases, to try something you've never installed in Tonic than in this. (And I hope there's genuine value in things like being able to share links to your experiments.)
I like tonic-dev, don't get me wrong. But the workflow for using a package in tonic-dev is too slow compared to having it in a REPL, at least for me. Might be because I'm using Firefox and the performance for tonic-dev is terrible there, but mostly because I feel more at home in the terminal than in a js-editor in the browser.