The past shapes the present. In this fact, simple but significant, we can see why the past is important. In an ordered sequence that goes from A to B to C, we can see that the nature of the entire sequence changes when one letter is removed. Also, there would be no "C" without a "B" before it.
The present relies upon the past to become what it is. This is true for individuals and nations.
One example of a practical implication of this idea can be found in parenting. If a person would like her child to grow up to be similar to her, she will need to provide similar childhood experiences for that child. She will need to have knowledge of her personal history if she wants to repeat that history for her child.