Why go back at all?

Thanks to a misunderstanding with a fellow imdb poster I have this new plot hole which makes any time traveling to "save the world" or "not change history" useless.

The usual understanding of time traveling says that if you go to the past you do not come back to the same future because of the changes ("Butterfly effect" - very good time traveling movie btw).

So if someone goes to 1937 to change history he will come back to the history he made it because of him/her being there. The actual timeline/dimension where he's coming from is not affected. That's why there are usually no changes yet before the person/people enter the 2nd machine.

So we see in this show they're coming back to a different timeline/dimension that is changed because of their doing. If they haven't gone back the changes wouldn't have happened and they would have stayed in the timeline/dimension they originally left.

Some say the timeline/dimension would cease to exist which tells us: "Ok. Why still bother?" We don't exist anymore anyway (except for the people who did the trip).

I hope I made that clear enough?

Ideas? Counter arguments?