> I don't really get why people need an online tool for this:
That’s rather condescending, and probably not true. Surely you must be able to think of reasons why someone would use a website instead of writing a one-line program.
The only reason I can think of is if this is something which one very rarely needs. Otherwise, I'd think those who regularly handle JSON files are able to write a one line program.
And the JS console was only an example. Other languages can also pretty print JSON trivially. E.g. python.