https://jsfiddle.net/ used to be also very popular, I see that it is still available, but haven't used it in a while. Which other alternatives do people use now days?
Anyway, the main uses case I see for these playground sites are:
1) small hand-crafted replications of suspected browser CSS bugs or the like, and testing possible workarounds. That often involves sending them to other members in the team (because I use Linux and Android, two colleagues in the team use Windows, and two using Apple products)
2) trying out some idea for how to make a particular component. Which is effectively almost the same as above, since that needs to be tested across browsers.
So I don't need bells and whistles beyond a simple text editor with code folding. That might not apply to everyone else.
Also, the exact same line of thought applies to js benchmarking sites, which is why I use jsbenchit (which is jsgist but for benchmarks). It also seems to be one of the few benchmarking sites without spurious de-opts of JS code, or noise from GCs triggered by the website itself in the middle of a benchmarking run.
[0] https://jsgist.org/, https://github.com/greggman/jsgist/
[1] https://jsbenchit.org/, https://github.com/greggman/jsbenchit/