Question

Why is it important to be aware of your audience's interests??

Answer 1

Your purpose in speaking is to persuade, inform, or entertain the audience. Your motivation is to get satisfaction from expressing your ideas and getting recognition or applause from the audience.
 In order to achieve your purpose, as well as to get the expression and applause you desire, you must satisfy the audience by speaking to them about something they are interested in. Thus, it is important to be aware of your audiences interests.

Answer 2

If you do not know your audience's interests you will probably lose their interest.