I've had lots of job interviews and "Why do you want to work for us?" or "Why do you want this position?" are the questions they always ask. It's actually quite a lame question, but for some reason it's always there.
In a job interview it's best to be yourself, relax and go with the flow...but that is the one question I would have a rehearsed answer for. Keep it short and snappy, along the lines of "I really want to work in this field, and this organisation seems like a good place to do it." To back this up slightly, have a quick look on their company website to see what they're all about in case they ask you to qualify that answer (i.e. "Well, in particular, I like the work you're doing with...")
If I were conducting an interview I would know if the person was qualified and able from their application form. I would rather chat to someone in a relaxed, informal setting just to see what sort of person they are. Some interviews feel like interrogations with people sat directly in front of you, firing questions at you from a clipboard. Some people perform well at interviews but that doesn't necessarily mean they'll be good at the job.
I had a batch of interviews recently and some of them were horrific (some even had me doing bizarre IQ tests and other such things) but after a few failed interviews I got offered a really good job at a nice place. If I had done well at the earlier interviews I would never have gotten that particular position, so you have to have a touch of, "If this is really the right place for me then it's meant to be..."
Having those bad, earlier interviews was also a good rehearsal. It probably helped me to get that job later on. By the time I went in, I'd had a few interviews (rehearsals) already!
Interviews are also a two way process. You're checking them out too. If they make you uncomfortable and don't seem to treat potential employees well, imagine how they'll be to work for!
Best of luck.
