What does it assess?
The interviewer is trying to assess several qualities, including your motivation, passion, and interest in the role.
How to answer it?
Do this exercise: List down at least 5 reasons and pick the top 3. You can think along in these directions.
In your answer, be honest and authentic about your motivations and interests. Avoid giving answers that you think the interviewer wants to hear, and focus on expressing your genuine passion and enthusiasm for the role and the company.