Whether you realize it or not, most videos produced for cinema or even high-end marketing campaigns have some sort of cropping on it, be it by design or with a specific purpose in mind. It's a common occurrence, and for the most part, when you wanted to use this kind of cropping you either had to make your own templates or scour the web in order to find what you were looking for. Well thanks to the folks over at PremiumBeat.com, they have compiled an extensive list of what they claim to be every popular video resolution in the world. All the way from 720 HD to 8K and creating crop ratios from 1:1 to 4:1 Polyvision.
Now one thing that should be remembered is that aspect ratios should be used as a storytelling tool and not something to be used to just achieve a look. So don't be afraid to try out some of the different looks in order to figure out what you want to use and how to use it well.
However, that being said, they can also provide some much needed help when you're trying to stabilize movement but don't necessarily have overhanging footage to use as a buffer. And no I don't mean Adobe Premier's cursed Warp Stabilizer, I mean proper video stabilizing where the entire frame is moved in order to steady the movement. Oftentimes this can cause gaps at the edges of the video where the footage doesn't cover and simply applying a letterbox can also be used to help maintain resolution without having undesired edges in the final product.
I personally hate seeing the letterbox matte on videos, sure they help while editing-but when I need a project finished at say a 2:40:1 aspect ratio I always crop out the letterbox
I just have to say, letterboxing aside, that was a crazy awesome video!
With few exceptions, letterboxing mattes is film school hack and a waste of screen real estate... if you need that "anamorphic look," then rent some anamorphic lenses (a set of Kowa primes are cheap and common).
If you're trying to get your camera B & C footage (i.e. GoPro) to match your anamorphic cam A, then it's understandable.