Sick and tired of SPAM in the forum

It looks like people post spam without any reason for that. What could be the reason to post a completely meaningless message that is simply a set of unrelated words? Or even worse, a set of what looks like words, but are just a series of characters? And the number of such posts in the convert-me.com forum continues to grow.

Of course I come and clean all such messages regularly. I have some visitors who volunteered to help me with the cleaning. I am very grateful they offered the help.

I've also set some restrictions to fight spam. One can't post a message with any external link in it if he/she has not registered in the forum. Certain known spam words (like "viagra") are detected, and the message gets deleted automatically. There is no reason to discuss drugs on units conversion forum, so it's definitely a spam message.

The question, that I can't answer is why do people post meaningless spam. I understand why they post links to adult sites, illegal software, etc. That's their way to make money. But what do they get when they post a meaningless message? Are they just happy they could get through all the spam filters without being spotted? It was not too hard, actually, the filters don't have an intellect and can't check the meaning of the post. What they try to prove with their posts? Why?