The drill shown in that video is a good way to get a temporary increase in your ankle range. A temporary increase.

Most mobility drills give you a temporary increase and not a permanent increase.

That means that a piece of the puzzling is missing.

The ankle joint for many people is pretty stiff and could use a bit more motion. I am not a huge proponent of banded joint mobilizations or self joint mobilizations with external loading, BUT the ankle is a joint where I think the benefits are large and the risks are very minimal (aggressive banded mobilizations of the shoulder may not be so good for everyone, for example).