You want to write ES6 code today, but your clients (= browsers) don't support it yet. So you use the transpiler: write ES6, compile to ES5, serve that.

It's the opposite of what Python did with the 2to3 tool, where you wrote Python 2, and converted it to Python 3 before running.

Why do you want to write ES6? Well, hopefully, every change ES6 introduces to ES5 is an improvement. I'd hope. So every different thing between them, you'd want to use, sort of by definition. That's the idea, at least :)