Your friend is taking her first psychology class. she comes to you saying, "i don't understand why we are studying the brain; i thought this was a psychology class." because of your background in psychology, your best response should be

This is because psychology is the study of the human mind and behavior. So, basically, the human part that is the focus of this field is the human brain. Due to its limitless areas of interest (conscious, subconscious and unconscious), we have to understand how the brain works to the best of our ability.

Hope this will !