Similar to HN in some ways. There is not much small talk happening here. And no obligation for anyone to talk about some preassigned subject.
1. it allows politeness without deepening the relationship.
2. it can be a breadth-first search of mutually-stimulating topics for in-depth conversation (which may lead to a deeper relationship if desired).
3. it can avoid accidentally offending someone with controversial topics or opinions.
4. is quick to disengage from, unlike a story or debate, so is useful as a conversational holding pattern.
HN doesn't really fit any of those. Sure, some comments will say "my kid does this", but I find they're rare on HN (and usually downvoted); in general, comments are adding new data or engaging in debate.