This question is really an interesting one, because it underscores how approaches to work shift as you age. When starting out, you need money, so you need a job. If you want more money, you need experience to get a better job. And this dynamic pretty much encapsulates the driving force for all applications.
"Frankly? Because I need money. I heard that you guys had money, and were willing to exchange some of it for time spent using a specific set of basic skills, which I've evaluated and decided I'm competent in. But also I blah blah blah complementary goals blah blah personal development blah blah respect your values."
If I were interviewing other people for jobs at 22, this question would never cross my mind. Of course people want to work here for money.
But as you get older, your reasons for not wanting to work at any particular place starts to grow. I don't want to work someplace where my skills stagnate. I don't want to work where my day-to-day tasks are too repetitive. I don't want to work someplace where they don't believe I have a right to a personal life.
So if I were interviewing other people for jobs now, the question seems very relevant. Why should I invest time and resources training you as our new hire? Convince me it's not a waste of time and money and that you're not going to be out of here the second you think you've got enough experience to earn more money someplace else.
I know people always say to concentrate on the positives, but it may be easier with this kind of question to focus on the negatives. Imagine the worst job in the world for you, and say how this job would be different. "I like how devoted you are to your Continual Development Programme. I like the diversity of the position duties, that there's a wide variety of activities I'll be responsible for. I heard that you had a policy to discourage after-hours calls that I really admire, because it I know it will mean that efforts I give to the organisation during my personal time will be really respected."
But I don't think a lot of people starting out spend much time thinking of things an employer might do, to which they'd say, "Well I wouldn't even want to work at a place that thought that was okay to do."