We've occasionally used it when discussing Python X vs Python Y behaviours across separate machines (I don't have 3.6 installed but 3.7, a coworker has 3.6 but no 3.7) and are the kind of questions you just want to quickly check, not create a new virtual environment (under Homebrew on Mac, good luck having 3.6 _and_ 3.7 if you regularly `brew cleanup`...) or spin a docker instance. The coworker also regularly tries Scala stuff there (he's had to code review some simple things to start getting familiar with the language), he seems to like it more than the normal Scala repl (which I use).

It's also handy for funny little languages you may not have (the APL version it has is decent enough, for instance, although I prefer GNU, as the free one)

Also, on an ordinary local repl, I'm never going to have to suffer through watching ads... which is where I see this sort of thing headed.

I can quickly run some code on my daughter's Chrome book. I like having that option.

I use it whenever I don't have a local shell available.

For example, when using an iPhone or iPad, or if I need to test something real quick in language X and I don't have X installed on the machine.