A website designing book that I've read recommends using alternate text for stills and videos. Basically to give a brief description of the media file. The reason for that is that SEO rank higher, since those texts are meant to support a blind person browsing or searching the internet. Basically ranks your page, image, video, etc higher since it becomes accessible.
On the creator's end of things, keywording makes archiving and databasing easier. Thus reducing some workflows and increases efficiency. For vlogs, if you don't prep ahead of time, it is still possible to be achieved after you get your shots.
