Instead of giving a generic answer to this question, focus on showing the interviewer that you have thoroughly researched their agency. For example, if you say that you want to work for their company because they treat substance abuse, that could apply to many other agencies. Instead, say that you want to work for their agency because they have a reputation for using an innovative approach to treating substance abuse with a higher success rate than other similarly sized organizations.

"I want to work for your agency because you are a client-focused agency that believes in offering exceptional customer service as your top priority. I also believe in putting my clients and their family members first while helping them overcome substance abuse. With your stellar reputation and high rehabilitation rate, I cannot think of another agency I would rather work for."