What are relevant and irrelevant answers

Something that is relevant to a situation or person is important or significant in that situation or to that person.

Sometimes it's because the question is ridiculous enough, or otherwise challenged, that an irrelevant or joke answer seems to be all that it deserves. Other times, the answerer has received an A2A, and feels that they need to try to respond, even if they have no real answer to give. And sometimes it may be because they simply don't understand the question, and think that they are giving a good answer.

the group received an extra pound of butter for christmas.

the annex was delighted to hear that we'll all be receiving an extra quarter pound of butter for christmas.