He had two purposes in writing his Histories. The first was to preserve the memory of the past. He believed that keeping the memories of great decades alive would enliven the youth. This also explains why Hitler only allowed propaganda to be taught in German schools. He did not want impressionable young Germans to learn that other nations and cultures were also strong and powerful.
History is full of commemoration and an attempt at explaining the past.
Everything that has happened in the world is and was important. Herodotus chose to concentrate his writings on war. He decided that epic poetry was not the way to go since prose was more detached and less emotional. He focused on human beings in history, moving away from religious explanations. History was no longer simply the whims of the gods, but history was made from human decision, reason, and common sense.