"What's your Purpose of Travel?" is the first question you are likely to hear from a consular officer during a U.S. nonimmigrant visa interview at most of the time. It's also very common to hear this at ports of entry when going through customs. Your answer will give the consular officer or CBP agent a first impression and all the following questions will be made under this impression. For instance, if you answer that you are going to a college in the U.S., the next question is likely to be "Which school and why you pick this school?" And if your purpose is to attend a business meeting, they will likely ask "What business do you do and what meeting is this?"
During the interview, consular officer assumes all the applicants are traveling with immigrant intent. They will ask different questions to see if you are lying on your purpose of travel. You must demonstrate this is really your purpose of travel during the interview.
To qualify to obtain the visa, your purpose of travel must be within the permitted purpose of your visa type. If you are applying a B1/B2 visa, your purpose of travel must be enter the U.S. for vacation, business, pleasure, or medical treatment.