Humans have been around on the planet stretching back to hundreds of thousands of years. However, recorded history only exists for less than the last ten thousand years. So we know of much less than 4-5% of all of history that has taken place so far.
Given this limited knowledge, it seems that human beings need to at least study this and figure out all the bad things that happened in the past. This will act as a sort of guiding beacon to avoid repeating the same mistakes and also in responding to similar situations which may come up.
For example, studying the effects of such varied events events that had heavy casualties could provide good insights into the kinds of things which can go wrong. The Protestant reformation, the Mongol invasions and the Taiping Rebellion are examples. This is especially helpful for risk management.