He is a rip in the fabric of time, and fate has a funny way of making sure she isn't compromised.
In that place between life and death, those who die because he lived are given a choice: continue into the unknown of the afterlife, or journey back to the time before their death with the full knowledge of what will happen.
Not everyone chooses to go back, but in July 1997, as Harry Potter turns seventeen and the battle between good and evil heats up in the wizarding world, more and more try to change their fate. They find themselves alive and in the same place they were months before they died, and they have a chance not just to save themselves, but to save the wizarding world as well.