Probably one of the biggest reason to read this manga is to get a deeper understanding on the personalities and get some side story on the players in the Program.
It does not lessen the maturity level of the series, though.
I have noted a lot of difference with the manga and the movie. I find it a bit interesting comparing the manga and the movie. I do try to be careful not to spoil some stuff…
If you’re like me, who found the story of the movie too fast, read Battle Royale manga.