Some ideas are:
- You branch off into another timeline and your actions make no difference to the previous timeline
- You’ve already taken said actions but just didn’t know about it so nothing changes
- Actions taken can have an effect (so you could suddenly erase yourself if you killed your parents)
- Only “nexus” or fixed events really matter, the timeline will sort itself out for minor changes
- something else entirely
Be a potentially energy-efficient way to exit a gravity well in a spacecraft if you could exploit that and it doesn’t require too much energy. Instead of launching a spacecraft, just send it back in time to when a point was no longer in that well.
EDIT: if the above conditions hold (it’s possible and requires less energy than launch), you also have an infinite-energy-production machine, because you can obtain more potential energy than you are expending energy to time-travel.
If you screw up the calculation, your time machine can also end up deep under the mantle of the Earth. That would be a pretty spicy way to travel.