Any candidate can read and regurgitate the company's About page. So, when interviewers ask this, they aren't necessarily trying to gauge whether you understand the mission-they want to know whether you care about it. Start with one line that shows you understand the company's goals, using a couple key words and phrases from the website, but then go on to make it personal. Say, I'm personally drawn to this mission because or I really believe in this approach because…" and share a personal example or two.