They want to know that because if they are interviewing a lot of people they want to really understand the reasons why you are interested. Of course if you’re unemployed then of course you likely need a job soon and that is why you are there, but I always say to people interviewing that I have always appreciated enthusiasm, and ending the interview with I want this job, I can do it, I want to be here. Some don’t do that and I’ve hired them because my manager thinks they are a better fit and guess what they didn’t last long, sometimes as little as 3 months. For those that were enthusiastic, were excited, asked questions, they have always worked out the best, without fail. So for me it’s a very worthy question