Once time travelers have conquered all the big challenges (i.e. kill Hitler, buy Apple in 1980, stop Skynet), they can go back and make sure Sir Isaac Namath discovers the law of gravity.
It's the 24th century version of jacking with Wikipedia.
>>ourman+(OP)
People keep going back to kill Hitler. But the resulting future is a butterfly effect nightmare result. So, people keep going back to save him. That's why he had so many close calls.