This is a further measure to stop spam robots which are still getting through. Spam problems originate from two main sources: The first and overwhelmingly worst is robots that post messages containing amazing amounts of links without signing up for a user account. They simply hit boards, hoping that they allow guests to post. I don't, but this doesn't prevent them trying to register. They somehow bypass the catchpa to do this - their creators are more sophisticated than I am at this stuff. You can still post text links to pages elsewhere of course; they simply won't be clickable. Users will need to copy and paste to use them.
What I am hoping to stop is the automatic registration process. phpBB allows users to include a link to their site in their profile. This causes spammers to register accounts on thousands and thousands of boards, boosting the PageRank of their sites. More on this later.