When submitting a link, sometimes I want to add a text in addition to the title. Usually I just add a comment in the discuss/comments section, but it'd be nice to have a field for that specifically.
This works especially well if you're submitting a link and want to add a remark about why it might be relevant for others to check out.
Adding a comment doesn't work because it can easily drift down the page. There needs to be a way to supply an initial comment with a URL that remains tied to it at the top of the page. It doesn't need to be votable.
As I use news.yc more I think this is more and more vital. Not only so the OP's comment stays with the URL, but that comment also needs to go into the RSS feed, otherwise I just get a load of one line titles which give little clue as to whether I want to read it or not. I end up wasting time on crap.
This works especially well if you're submitting a link and want to add a remark about why it might be relevant for others to check out.