Originally Ace just passed by to see Luffy, gives Luffy a paper, and left. In the anime, my guess is the animators, studio all who were in charge of animating ONEPIECE back then. They felt it would be cool if Ace accompanied the StrawHats for a little while.
Ever since Ace appeared for first time in the manga, he has shown how cool of a character being. Big brother of the main character? That sounds cool, right? However, his existence in the anime Arabasta didn't change anything in the series of events.