The reasons spammers want to register on a forum is to include their website link in the registration, that link increases their rank in search engines. So even though most people never see the registration, the search engine crawlers do.

You also cannot block spammers from registering, even with the number-letter fields during registrations. Some spammers use porn sites to trick people into manually entering those codes and info to enter their site, when if fact they are really registering on your site!