He is showing you how you can nest rules within rules. Why do this? Because you type far less repetition in your code by not having to repeat typing the same class, thus using one rule for the selector.

So it isn't more typing like you said, imagine saving the typing for longer selector names than the example in the video.

The output isn't going to matter, when you are going to compile/code it though sass anyway. Furthermore, you want to limit the level you nest selectors (no more than 2 levels if that).