Example: “I would like to continue developing my marketing expertise as well as my leadership skills over the next several years. One of the reasons I’m interested in working for a fast-growing startup company is that I’ll have the ability to wear many hats and collaborate with many different departments. I believe this experience will serve me well in achieving my ultimate goal of someday leading a marketing department.”
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.