They're job ads from YC companies, as mentioned in the FAQ: https://news.ycombinator.com/newsfaq.html

You're joking, right? On tech topics, every third comment is somebody pointing to their company. Sometimes they mention it and sometimes not but it's still advertisement. Not even talking about people spamming their site.

We have ads here, they just have to be carefully crafted infomercials with a high payoff for the advertisee, enough that it could not be an ad ...

Basically, "Mission. Fucking. Accomplished." - https://xkcd.com/810/.