Youtube is the best way to add video to your site. If you upload it directly what ever file type and size you upload is what everyone will have use. So if you upload a full HD video to your site everyone that views it will have to download that full size hd video. That’s fine on a computer with an unlimited internet connection. But on a small cell phone screen there’s no reason to download a huge video for such a tiny screen. And maybe the internet speeds are not the best. If it’s uploaded to the site that one file is all you have.
Now if you upload it to youtube they are able to covert the video to many different sizes, quality and file types. If you’re only going to have one mp4 is the best. But maybe the viewers phone plays WebM videos better or some other file type. Youtube will provide the type of file the device prefers. If you’re viewing the video on a computer it’ll give you a full hd video. If you’re on a phone it’ll give you a high quality video sized for your phone so it looks just as good but it doesn’t use as much data. If you’re on a phone with a slow internet connection it’ll give a lower quality video that will download fast enough for you to watch the video without buffering constantly.
And on your side it’s just easier. You don’t have to worry about file size or type. Just upload the video to youtube and they take care of the rest.