> Should I answer anything else besides "I'm looking for a job and this position matches my skills"?

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.