I have noticed SM idols do it for a while but didn't know what it is tbh and now that I think of it, I like the idea. It's better than teasers and it prepares you with the concept and feelings the artist is trying to portray in their work.
I think it is just an SM thing. Yes, it can be a little better than the teasers. I personally don't watch most of them. I've only watched Kai, SHINee, and Wendy's. Usually, I'll just wait until the album snippets BKA "medley" vid is dropped. That's pretty much the vibe sampler for me.