As a story element, it's very difficult to get right, often undermining the seriousness of a narrative by providing an easy out or redo for actions. Further it can get very complicated quickly and a poor writer will not communicate it well to an audience.
As a scientific endeavor it's consequences are pure speculation and would be greatly varied depending on the true makeup of spacetime and function of the machine. A traditional storybook time machine leads to several interesting problems such as time paradoxes where causality can be circular or possibly getting refugees from a future catastrophe.
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