To answer both questions of "what to write" and "why they ask for this," I will quote an answer from @badroit to a related question (which deals with writing a research statement):
You're trying to demonstrate why you and your background are a good fit for a summer school and that you will benefit greatly from that school.
The general idea is the same for writing a "why I want to attend" statement: you want to show the organizers of the summer school that they won't be wasting their time and other resources on you.
So, the two things you'll want to make sure your answer conveys are:
- That you are genuinely enthusiastic about the topic. Explaining (as you suggest) that the relationship between biology and mathematics is something you are very curious about/interested in is a perfectly reasonable thing to write. You could also mention related coursework or projects you have undertaken, to further demonstrate your interest in this area.
- That you will benefit from the summer school. Discuss relevant goals that you think this school will help you achieve; for example, your intent to take graduate-level courses in the subject as a senior, to pursue independent study/research in this area, to go on to graduate studies in biomathematics, etc.
Finally, if there is a professor in a related area with whom you have some relationship (e.g., you took his/her class), you can ask them to review your summer school application before you send it in. This has the double benefit of (1) getting expert help with your application, and (2) cultivating a relationship with faculty in your field of interest.