-Savestate right before a job interview so it would be like practicing and doing it for real at the same time.
-I would go to an amusement park, savestate right after getting in, ride what I want to ride, and near the end of the day, load state back to the beginning, and ride the rest of what I wanted to ride but didn't have time for the first time. Same for big carnivals.
As I am a rather scientific guy, I would assume that loading a savestate would recreate the exact same universe as when the savestate was created, including my brain and therefore my thoughts and memories, so I would be unable to gain any obvious advantage from such an ability.
I would further assume that the universe is fully deterministic, including quantum physics; we don't know the rules, but I do believe they exist. (We will never be able to predict the universe, as a perfect universe-simulator would require more than one particle of memory to represent one simulated particle, alternatively would be identical to its simulation and therefore not be a simulation at all.)
This means that all events between state saving and state loading would be perfectly repeated; including loading the same savestate again. The end result of this would be rendering the universe devoid of any further meaningful activity, and it would be the equivalent of any other universal doomsday device. I don't even know if it's possible to differ between a savestater and an instant universe terminator.
Alternatively, if my assumption of determinism is false, and the many-worlds interpretation is true as well, it is possible that many, or even most, of the alternate universa(*) have ended already, and we're really damn lucky, but that's implausible. Instead, it is more likely that many of our clones may have ended already, and we're here just because we're still here. It's the same reason we live on the only known inhabited planet - the others are uninhabited, therefore we're somewhere else.
To return to the original question, a rewinder may bring useful benefits to me, even if many-worlds is false, assuming the universe is not deterministic. Since the quantum phenomena will go differently, the world will not be identical to last time; however, I must very carefully set the rewind time, as if I set it too short, I will redo it forever (I won't know how many times I've done it, so I won't tire of it), and if I set it too long, I (as well as the rewinder) will cease to exist.
So the short answer is: If I had savestates in real life, we would know a whole lot more about the universe; however, I would be unlikely to use it, as destroying the universe is unlikely to yield any desirable effect in any eventual afterlife I may end up in, if any.
* Isn't 'universa' an oxymoron, anyways?
-------------------- <blm> zsnes users are the flatearthers of emulation