1. To see if you have done your due diligence. If you can't be bothered to due diligence your next JOB, chances are you are a slacker/drifter/suck at research (resourcefulness), none of which is a desirable trait. 2. To see if you'll start bad mouthing your former/current employer. You can tell a ton about an applicant from their complaints. Also good to get some dirt on the competitor. 3. Genuinely to see if there's a good fit.
1) They want you to beg. 2) They know that employment is a hierarchal game and your answer should show that you know how to be obedient within the rules of the system regardless of whether you're an alpha dog or just a regular conformist employee.
I don't ask the question, I can gauge the level of enthusiasm of the candidate without explicitly asking it. But I don't think it is an outlandish question.