Spam is a big problem these days. Spammers use advanced bots that can post thousand of spam messages on your forum, your blog, or any other type of site that allows the user to post messages or comments. The bots are becoming more and more advanced.
A few years ago it was easy to stop this type of spam using methods like CAPTCHA but now the bots have sophisticated ocr algorithms that can easily detect the obfuscated characters in an image so that's not a protection anymore.
If you run a forum with phpbb you will have to install a few mods to protect it against spammers. If you don't do anything you will get hundreds or thousands of spam messages a day, you'll want to bang your head on to the wall trying to keep up and delete the messages and your forum users will most probably leave your forum before you'll manage to get rid of all that spam.