Even if it is an amazing photograph or a well thought, shot and edited video, there is still a lot of benefit from adding a good description to it:
- As you mentioned, it helps the viewer have a rough idea of what to expect, especially potential followers who don't know you yet.
- Search Engines can't really analyse the content of an image/video (or maybe they can now, I'm not sure) so in order to bring more organic traffic from outside the Steem ecosystem, your best bet is text content.
- I saw some vloggers adding a transcript of their video in the post body and that is absolute gold for me. I spend a lot of my time Steeming from an iPhone in the train and I can't always watch videos, so a transcript allows me to know what's going on without watching, especially when the videos are just vloggers discussing a subject which does not really require you to actually watch.