This feature is your first line of defense against spam bots. Most spam bots are unable to get past the registration page when this is set to 'Advanced'. This should stop them before they are able to register. The second line of defense is validation, which can be enabled lower on this page under "<i>New registration email validation?</i>".<br /><br />
If you continue to get spam registrations, you may wish to ban common spamming IP addresses, names, and emails using the <a href="#" onclick='opener.ipsclass.location_jump("{base_url}&section=content&act=babw&code=ban", 1); return false;'>Ban Filters</a>.