I guess it ends up being trub in the end, which we don't really want in the ferementer.
I think that is the heart of it. You generally want to keep the hot break material out of the fermentor (though you will get different opinions on this).
Depending on your equipment and process, you can leave some of that material behind in the mash tun, you can skim some of it off at the beginning of the boil, or you can leave it behind in the kettle post-boil (or any combination of these).
In the case of the video you posted, skimming all of that foam was probably necessary to prevent a boilover since the kettle was so full.