I have no idea. I just spent a few minutes brainstorming for reasons why someone would use the down arrow. In general I have learned that if I can come up with something (even if it disgusts me), then there are at least a few people out there who will do it.
I also had at least one option that I considered adding to the list but didn't because I didn't want to suggest it as a possibility.
Of course they do. Or if two people are disagreeing through multiple comments, you can tell as each person comes along as every one of your comments will get a downvote (and, I'm sure, the person you're arguing with is probably earning an upvote). So you learn that if your opinion isn't popular, do no reply multiple times or you'll "pay the price" multiple times.
Even if your comments are long transparent, people need to "exert their opinion" and penalize you in aggregate.
There's a conversation where, in response to someone hailing an app developer for linking to competitors, I observed the benefits that such an action brings. I had responded to a couple of replies, and could tell as each person came through because they didn't just skip that no longer relevant, transparent conversation. No, they had to read it and then gang downvote. Which...what's the point?
Wait, do people actually do this? (outside of Reddit, anyways)