I too, would personally prefer a command line REPL. But sometimes you are interested enough in a thing to try it out, but not interested enough to install things. Coq, in particular, is a pretty brutal installation process if you aren't already an emacs user. These web REPL's are great for reducing the friction necessary to try something out, and thereby getting people more interested. This one in particular is a big win for convenience over existing tooling.

I will add, I think it can be very disheartening for people who put a lot of their own time into making something happen and then having a random commenter say, 'Yeah, but I would rather have had something else, that wasn't at all the project you set out to make.'