By learning history, we make sure that history does not repeat itself by learning from the mistakes of others. Learning about the past helps us to develop as a society and come up with ideas for the future, using the information that others have given us. It's also very important for us to remember where we came from and what our history is. History can help us understand more about other cultures and why they think a certain way. By learning about other cultures it helps us appreciate what we have and helps us to combine cultures, getting the best of everything. When an immigrant comes to the United States a lot more native-born Americans are willing to accept the immigrant because they know about that culture's history and why they might've come to our country (push factors, pull factors). We also can keep in mind that our own relatives were also immigrants at one point, and therefore we have learned we should try to help them feel welcome in our country, rather than condemning them.
Also, when visiting another country by knowing information about that country beforehand, you know what to expect and you can broaden your horizons as a student of the world. By knowing more about your allies and your opponents' countries, then you might have better insight about what you should do when coming in contact with them.