This is a question which comes up in all job interviews. Avoid answers like “I need a job” or “I need more money” or any response which benefits you.
The employer is trying to see how well you will integrate into the culture of the workplace.
I recommend you research the company’s website and publications first and find something unique which has compelled you to apply that fits directly with your personal beliefs, values or expertise.
A good answer could be:
I heard about your charity division which trains disadvantage kids into a career in software development. I personally give back to my local community too by volunteering my time to teach kids to code. In a way I felt your company and I share the same fundamental goals and values.