Doesn't pretty much any browser have this? If you're doing JS dev, chances are you have a browser open.

True. The appeal for me is directly manipulating the code, and getting feedback. The browser console is laid out in a call response model, and you are unable to edit previous entries. You have to retype them in, or hit up on the keyboard to find the previously entered function. It is nicer to just manipulate my code and evaluate it right in the editor. It isn't a huge difference, but it has been helpful for me.