Earlier quoted context omitted.
I don't think this is a good example of the value of these things. You can just as easily do that same thing with advanced text editor features. Sublime for example supports multi-cursor editing. Just hold alt+shift+arrow keys to add a cursor, then type in the brackets you want. Ctrl+D can be used to select the next occurrence of the current selection with multiple cursors, built-in commands from the command pallete…
Multiple cursors wont do what the parent comment is talking about without a lot more work.
going from
a: 'one',
to a['one'],
just requires you to add two brackets and remove the colon. With multiple cursors you can do that exact same operation for all lines in a few keystrokes.