If you google " How do I get to use my dried out bottles of nail polish again", the Internet divides itself in two groups. One says: add a few drops of nail polish remover to the bottle. You have it on hand anyway and it will thin the polish enough to use it again.
The other group, that sounds more expert, says that adding remover will spoil the remaining polish as the remover’s job is to break down the chemical structure of the polish.
Nail remover usually contains acetone,
Nail polish thinner seems to contain different chemicals then remover, but I can’t find out (Googled it for 10 minutes) which chemicals and which properties they have.
Throwing out good but dried out bottles of nail polish is a waste of money. Besides, all those bottles add up to an environmental hazard. So such a nail polish thinner might be a useful product. I just hope that some Beauty Buff Doper has the Straight Dope on it.