There are 4 kinds of comments:
* Mention of someone, to make him/her aware that job may interest him/her
* Correct the person who posted on the job on a typo
* Mocking the job. Often a big circle jerk we have to moderate
* Someone saying that the company is awesome they have worked there or know the founders, ...
I find it quite interesting however, it's never really bringing anything valuable to the job post. The comments are never about asking what are the use of X language, how are the teams, ....