We do sometimes place stories that we or a small number of story reviewers think the community might like. That program is described at https://news.ycombinator.com/item?id=11662380 and the links back from there. Such stories get lobbed by software randomly onto the bottom half of the front page, whence they fall off in a few minutes unless people upvote them. The purpose is to make HN more interesting (because https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...). Many good submissions fall through the cracks and this is a way to give some a second crack.
Anyone who knows of a story that deserves a second chance like this should email us at firstname.lastname@example.org and we'll consider throwing it in that pool. It's ok to ask this for one of your own stories, but it's better if you just ran across something and think it's cool.
By the way, you can see a partial list of these at https://news.ycombinator.com/invited. Those are the ones that were too old to be lobbed directly when we saw them, so we emailed the submitter and invited them to repost it instead. It's on my list to publish a more comprehensive set of lobbed stories.
There are two other types of placed submission that HN does as a way to give back to YC for funding this place: job ads (https://news.ycombinator.com/jobs) and Launch HN posts for startups (https://news.ycombinator.com/launches).