Guess for what?
Of course for rolling time back ^^

I see it in this way: we set value something like
time_memory = 3;
so engine remembers, what happens in last last 3 seconds and can roll 3 seconds back, if needed(if time_factor is negative).
By default time_memory = 0, as some users may not to use this feature, so we needn't slow down their systems.

You can not run a game backwards, but you can already roll back to a certain time with the game_save/game_load feature.

Yes, i cant now, but i'm speasking about future features

Also, i can create array of states and positions for every entity, but i'm talking not about this, but about engine feature.