letterbox

I am currently making shortfilms on youtube with my gopro, i've seen many other youtubers using letterboxes. it looks pretty nice butmy question is what do you guys think about these letterboxes, are we who's doing shortfilms gonna use these or not?

Why are music videos on YouTube LetterBoxed? Not all are but recent ones I've seen created by YouTubers are. Here is a picture to show you what I mean. LetterBox is only for if the video doesn't fit the frame right? Am I missing something. Should I be doing this or is this only an option? Just...