Its a REPL[1]. It aims to be host environment agnostic, where you could play around with the language itself (Its accomplished by sandboxing the language in a web worker, and this is also done for Python and Ruby). Try firing up v8 or nodejs shell and do an alert.
There is an added security value in that, were you can be sure that code shared by someone else won't crash your browser.
We'll be releasing courses and tools soon that aims at teaching JavaScript and the DOM where you can go crazy with alerts ;)
You could use console.log, console.time, console.read etc.
IMHO one should use http://jsfiddle.net/ or similar sites to learn javascript.