In time travel stories -- leaving aside those in which you spawn alternate words -- there are worlds where you can't change the past, because whatever you did when you traveled in time is what happened then, and there are worlds where you can change the past, and so the universe.
The first, in my general experience, produces cheerful, even comic works, like The Hitchhiker's Guide to the Galaxy and The Technicolor Time-Machine . The second produces, often, such tragic works as most of Poul Anderson's Time Patrol stories.
Perhaps a world in which we are safe from doing damage is like a great big playpen, designed to protect us.
Or perhaps I have read too few works, or have a biased sample, or am misremembering the trends. . . .0:)