We got hit with a bot yesterday - new registration under the username "tomandjeff" posting nonsense. Banned the username and enabled the email registration verification. A new member now has to respond to an email before they can post.
in this case it's a computer that searches out forums, generates an account name and begins posting a scripted message usually inviting people to click on a link to a website. Sometimes those websites contain viruses, spyware, or other nasty things.
By using email verification it inserts another step in the registration process and can confuse the attacker so it gives up and moves on.