"Why do we have to learn this?"

When a student (high school) asks me that question, I have a few different responses.  I think my favorite one is to tell them that they might never use this particular math equation in their future, but what they will use are the habits of mind they are developing as they learn how to solve math problems.  Math teaches you how to think logically and creatively at the same time.  The "aha" moment in solving a math problem brings together the synthesis of logical thought and an intuitive leap of genius.  It's an important skill and it can be useful in a large number of endeavors.