Note: this post is written from the context of SE sites in general, and may or may not apply to my activity on pets.SE specifically. In particular, questions on SE sites for math, chemistry, and programming have clearly "correct" answers much more than those about pets, psychology, or even advanced physics.
I might answer in a comment if any of the following are true:
- I'm not sure my answer is right
- my answer is speculative, e.g., "based on statement x from your question, it seems logical that inference y", but I have no reference or experience to cite
- my answer is very incomplete, e.g., "only when it's raining" is the full text of my comment
- my answer is too narrow for the question, e.g., "well, if specific situation q is true, then answer p" when question obviously includes other situations too
If and when I do this, though, it's because the question interests me enough that I feel compelled to offer what little I have at the moment. It might be a seed for someone else to turn into an answer (either because I don't have enough to offer without research, or because I want to let someone else get the rep for turning my drive-by comment into a good answer), or if I check back later and nobody has done so, I may decide to turn it into an answer myself.
Another possible situation is a question from a beginner in the topic at hand, such that the answer is either common knowledge among non-beginners even if they aren't experts, or is extremely easy to find (http://bfy.tw/1EYM) - I might "drop a hint" in a comment for someone else to pick up and build an answer from.