This question is designed to gauge the root of your willingness to work within the government. An employer wants to understand why you want to work for the government specifically, as opposed to other areas of society. You should avoid answers related to pay, benefits or time off, and instead center your answer around the foundational reason for government.
Example: “I’ve always believed that service-related work is the most rewarding. Knowing that my job is part of a larger initiative to maintain and improve public life keeps me motivated.”