They're usually by newbies to see if that submit button really works.

They probably should be banned automatically somehow, they add a lot of noise to the new page and sometimes they last quite long.

Flagging them helps getting rid of them, the more people flag the sooner they're gone.

Also, see http://news.ycombinator.com/noobstories

Another explanation is that link spammers use something general to see if they can post 'any link' without further credentials, usually the same author will follow up with a spam link a couple of minutes later.

Some hours of day the amount of spam is quite amazing.

With HNs rise in popularity this will probably get worse, so getting rid of the spam early is an excellent way of discouraging it, and that includes such 'trial balloon links'.