Why do anime end with spoilers? Is there any rationale, or at least historical reason, why many anime end their episodes with spoilers to the next episode? Even those that rely heavily on plot twists, it's very surprising and one needs to stay sharp to skip it early enough (only to discover at the end of a certain anime, that it actually contained additional scenes and not spoilers...). Unlike with "previously on X" recapitulation I see no benefit in this.
It's to get viewers hyped up to see the next episode.  

If they do it right, they don't give all of it away, and they leave the viewer with a sense that's there's more to see, and that they need to watch the next episode to figure out what happens or even how it happened.