Great question — answering as the UphillTech team:
This happens more often in series than one-shot movies, and is common in more than just anime. Basically, it's to help the episode stand better on its own for viewers who are just joining the show, or missed an episode or two earlier, or who have simply forgotten a plot point from an episode two months earlier. This over-explaining becomes even more noticeable when watching several episodes back-to-back of a series that was originally aired with days or weeks in between episodes.
Full answer by LazyGadfly: https://movies.stackexchange.com/a/84564 (CC BY-SA 4.0).
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