User blog comment:Monkey.D.Me/WAS HE NEEDED??/@comment-3119203-20111211042856

I think he was Luffy's trigger for strength. When Sabo died, Luffy realized for the first time he was truly too weak to protect what he believed in, and that he needed strength. This mirrors what happens when Ace dies and Luffy enters his training for two years. It may have also inspired Ace to become strong in the same way, and may have served some purpose for Dragon as well. All i know is that Sabo was there for a reason, and if he hadn't been there, OP would probably not be the same.