Wikipedia:Don't Know, Don't Care

Sometimes certain users may take the focus of a particular discussion off track by asking lots of ancillary questions that ignore the central point. It's ok to tell such persons that you don't know and don't care what the answer is.

Isn't that kind of rude?
Not if it's the truth. Persistently asking irrelevant questions can derail discussions because users lose interest in participating in discussions that are unable to maintain a focus on the core issue. This is a growing problem on this project and should be resisted. (If their questions are content-related as opposed to policy related, you could always direct them to the reference desk, where such nonsense is actually tolerated and even encouraged)

Well then, couldn't you find a nicer way to say it?
You could always try "I am not aware of the answer and don't care to find out" or whatever phrasing you prefer that expresses the same sentiment.

Why do they do that anyway?
Hard to say. Sometimes, they ask a lot of vague questions hinting at some particular situation that they are reluctant to clearly identify for whatever reason. Other times they may be deliberately trying to derail a conversation if they suspect they will not agree with the likely consensus that will result from it. Or, they might just be immature and prone to asking lots of irrelevant questions. The reason doesn't really matter.

When not to use
When they are asking relevant questions clearly related to the central point of the discussion. (Or of course if you actually do know the answer...)