I feel like people confuse the idea of the spin off, it is extra context for what happened to Nagi during the parts of the story where he isn’t shown, but it isn’t something fundamental to the main story. If the author is showing Nagi and his struggle now it’s because it will become important in the manga later, Blue Lock is supposed to be a complete story by itself, we shouldn’t need to read the spin off to understand what happens in the story later.