Harry Potter and the Prisoner of Azkaban

Harry and Hermione use the time turner to save Harry. Now that Harry knows about the magical existence of the artifact, why wouldn't he just use it to go back and stop Voldemort from murdering his parents? Wouldn't that just be the easy solution?

The Dark Knight

The Joker got thrown in jail so he could mastermind a distraction where Batman and Commissioner Gordon race around time. He did all of this just so he could break out of jail when no one was looking and kill the mobster who was getting in his way. Couldn't he have just done that without putting everyone else all over the place?