It is a recap episode of Episodes 1 to 11. It's up to you if you want to watch it or not. Whilst during the anime's run, it's considered broadcast filler, meaning, they broadcast this a week after Episode 11 before airing Episode 12 a week later, it can be valuable for some people to watch this prior to watching Episode 12. Maybe if they're bingeing the anime, it's not as valuable, but if you're watching an episode or two per day, it's gonna be valuable. Believe me, I love that it helped me recall important events from previous episodes before I watched the finalepisode.