The question is a litmus test for whether or not you're passionate about programming. That may sound unfair ("Why do I need to be passionate about this? It's just a job...") but the field we work in is constantly changing and the only way anyone has a hope in hell of staying abreast is if they really love it. So if you manage to keep tabs on the industry through your colleagues and you're excited about what you're working on at work, then the personal projects are much less important, but you need to demonstrate that.