Purpose of spam?

Hello guys!

I got a site that right now is under attack for spamming.

The site got a form where the spammer, unfortunately, can post topics. I guess I need to put a captcha or something.

But I can not figure out the reason for this attack.

They just put things like:

"US dollars http://www.xxx.com/essays-on-emotions aliases forever essay .com doubtless prop  Aston had hoped to sell up to 4,000 Cygnets a year..."

"Your cash is being counted <a href=" http://xxx.ky/performance-reports/ ">buy accutane in uk</a>  Both sides of their skinny shop have windows that open up to the street..."

What could they gain with this?

Any comments are most welcome.

Btw I have masked the url if they might be virus carrier.

Best regards
Fredrik