It depends, there are two kinds, internet-wise.
There's a good bot, that searches the forum for posts. (The ones at the bottom). These could help people find you through that search engine.
The bad bot though, is a program created by an advertisement company, to go and post adds around wherever they can get too.
Then sometimes, it's actual humans that spam. It's kind of like those collect calls. They usually do it for their company, as to not get in trouble. Some of the humans spammers do it, just to be buttholes. Their usually the overweight guys, who have no life, and sit at home at the computer all day hacking people/spamming forums, because they got nothing to do in their parents basement.