It works exactly as you think it would, with the twist that sometimes, for reasons unknown, the game cannot be loaded and it gives the ominous "internal error". If you increase the number by 1 or 2, it likely will load that game (but not help you with continuing that original game).
It's used for example to look later at a game you played to see what you could have done better, or what your opponent had in hand in a critical situation.
I use it to check bug reports. Often there are circumstances that the player forgot that explain something that at first sounds like a bug.
