Answer: It tells us what will happen again if we do not learn from the past.
We all need to be informed of slavery, the holocaust, evil leaders and regimes, and man-made environmental disasters so we can avoid having them happen again.
Ignorance of the past is very dangerous.
Intentionally revising history to say there was no holocaust (or to revise other shameful periods in history) is even worse because people won't learn from what they do not believe in.
If we look at how technology builds on itself we can postulate to some extent from the progression where we will be in X number of years. Looking at the history of the automobile, medicine, or any other field, we can make reasonable, educated assumptions.
Some assumptions in medicine, for example, are that we will have unrejected organ replacement therapy, a greatly extended life span and nanotechnology.
Step-by-step explanation: