I am super Confused about Demon slayer Mugen train 🤷‍♂️

I am confused about demon slayer mugen train, if they are going to recut this movie and make it into a 7 episode series, what was the need to make the movie in the first place?
demon slayer season 2
mugen train arc
red light district arc
tanjiro
rengoku kyojuro
akaza
anime cloud
badal yadav
bnftv anime
#demonslayer
#animecloud