Obviously it could happen for any number of reasons but my guess is that the most common reason is that someone encounters something for the first time and thin

Obviously it could happen for any number of reasons but my guess is that the most common reason is that someone encounters something for the first time and thinks "this is really cool" and wants to share it with someone, and then they think "I know, HN will love this!" and then they post it, and then they either get distracted and wander off, or they see everyone posting "why are you posting this, it's been around forever" type of stuff and they decide they have better stuff to do.