If you're hiring, why do you ask that question and what do you expect to see as an answer?
If you're an applicant, what do you usually answer?
When I've been the applicant, I always have an answer to this, though. Before I've even applied to the company, I've already done at least some research on them (even if it's just reading their website) and know the sorts of things they do.
There's something there that made me decide to apply at that particular company. I'm not just firing resumes out at random, after all. Perhaps that thing is small -- they use a particular technology that I'm interested in, or maybe it's just that I think what they produce is cool, or maybe just that they're located somewhere I want to be, or that they're in a segment of the industry that I'm intrigued by.
One time, as an example, I applied at a company because some close friends had worked there in the past and had great things to report about it. So I said that (along with how what they do intersects with my skillset) and elaborated on what I was told about the company that appealed to me.
In any case, whatever that thing was is the core of my answer to the "why us" question. I don't try to pad it or flatter the interviewer with how incredible I think the company is. I just talk about the thing that made me choose to apply there.
In reality, as another response here said, what the interviewer wants to know is that you have some idea of what the company is about and you have some real interest (even a little bit) aside from a paycheck. Your answer should demonstrate that.
Yes. If someone at a company is asking you this question at all, they aren't cynical realists that expect you to answer "because I need a job" or "because I want money" even though those things are the truth. People who like those answers just don't ask that question in the first place. You need to answer with some bullshit about how their mission really speaks to you or about how you didn't feel like you made enough of a difference to the world at your last company and believe you have a better opportunity to do so at this new place.
I actually try instead to work the conversation around rather than just bluntly coming out and asking a dumb question like that. But that's how I interpret it when the question is asked of me.
I don't rule people out just because all they're looking for is a job. It's just nice to hear that they did some kind of research before showing up.