The classic answer is: "To not repeat the mistakes of the past." But this does not always work, as humans constantly repeat the same mistakes. There have been many financial scandals over the years, such as the one that led to the Savings and Loan crisis in the 80s and more recently, the banking crisis in 2007-8.
It is important to research in order to find out not only what happened, but how it happened and who was involved. History is not only about the people you read about, the "greats" in you textbook, but the people who would have never had a chance to be in your textbook if they had not done something simple to change history. An often used example is Rosa Parks. If she had given up her seat and stood up instead of staying put, no one outside of her family would probably have ever heard of her.
Historians also need to research history in order for facts to be facts. If you look on the Internet, people often use figures from the past to propose a certain political belief that benifits them. Without historians, no one would be able to make sure whether someone was lying.