This is another way of the interviewer asking why they should hire you, over any other applicant. Think about the skills and qualities that you bring, which others may not have. Perhaps its your counseling style, a transferable skill, or even the fact that you speak 3 languages! Explain the trait and how it makes a difference to your patients.
"I believe that my best asset is the fact that I am a huge proponent to continued education. When I received my Masters' in Counseling, I did not stop there. On average, I take 3 additional courses or workshops per year."
Entry Level answer example
"My best asset is in my critical thinking skills. I'm able to make quick and clear decisions that make an impact in times of crisis."
Experienced answer example
"In addition to being a seasoned addictions counselor who has trained other counselors, I am also fluent in 3 languages: English, French, and Spanish. My linguistic skills have allowed me to help a wider range of patients during my career."