They might well know that it isn't a good fit for the site, but they likely still want to help the person asking the question, so try to pop in a helpful answer before it is closed.
I have answered questions before and voted to close them at the same time. It might not be a good question (ie, not a good fit for Q&A) but it still might be a good question.
In the question you linked, yes, it isn't a question that works well on SO, it is (sort of) open to interpretation and there isn't a few lines of code that will answer it - but surely you don't look at it and think Bah, what a waste of time this was... Some peep out there is asking for help, and he is getting it. In that case, I am glad that it was answered by folks with loads of rep, the answers probably had more insight into it than someone who started programming two weeks ago.
I would personally be much happier to answer a question like this and possibly really help someone than find a syntax error someone, or write a quick answer for a code beggar who can't be bothered to do it themselves.
Lastly - and this is from a game theory point of view - you will probably find that a lot of the high rep users will answer pretty much every question that pops into their queue, whether it is a good one or not. I mean, someone doesn't get to 63k rep by being picky and choosy with what they answer right? :)
Really Lastly - to back up the comment about getting the message below: I asked this question on SO before I really understood the format there. I still think it is a good question, it got voted to a +10 (some downvotes included) and it got some utterly fantastic answers. Having that question closed by a mod as it didn't fit the Q&A format taught me a good lesson - SO isn't the site for these. In this exact example, probably programmers@ would have been better, and I know that now, but I learned it by having it closed. Until that time, I thought it was a great question to ask.