Reason for watching: The extra scenes not included in the main anime series.
TL;DR: Watch for the additional scenes on Seijoh at the end of the movie if you want to know more about them after their match.
I absolutely love Haikyuu!, and I have no qualms re-watching every episode of the anime series to recap the story. I have always avoid recap movies as I get more kicks out of watching every small detail rather than just watching all the major developments.
That said, I adore Seijoh as much as Karasuno, and the end of season 2 have always leave a bittersweet taste in my mouth as I felt that there was no proper closure for Seijoh's match from their POV. The movie have additional scenes on what happens in Seijoh after the match. The fact that they are not just another "side character" team, but a proper rival team that deserves a bit of insight on how their emotional and psychological state outside of the game is like really showcases the beauty of this anime.
I am not a manga person, so the additional animated scenes to tell us a bit more of the rival characters that I have grown so attached to really brings me so much comfort. hahahaha.