I would also be in the camp of optional additional field when posting a link.

Here's an example post.

• With this post, I wanted to post a link as well as an explainer about it, out of respect for the readers time. And to help identify if it is worth a click or not.
• Yes it could have been a discussion post, but that defeats the purpose of having a clickable link up there.
• Instead, I had to post to SN before drafting a comment. That comment took 10-15mins to create after the post went live. Potentially lost eyeballs.
• Yes the 1st comment could have been drafted in markdown off the site, but it would have been much more convenient to have drafted it together with the original listing.
• It is actually a preferable experience for the reader to get a digest of what the link is about, if they want to click on the title.
• It doesn't need to be mandatory but the absence of a description field makes most posters not even consider creating a comment of any kind.
• It is then up to the community to fill that vacuum of summarising the link or providing context. Is that the intention? I'm not sure it is.