It is more an antispam thing, it attempts to prevent spammers registering for the forum, and editing their profiles to be full of links...if they don't post, then it is hard to keep track of.
For me though the main reason is the blocking of Personal Messages. Since someone needs 20 posts before they can send a PM, spammers cannot register, and start spamming everyone through PM's...
The only problem is that when the do register, they can still post...until they are caught that is...though generally the mods are fairly quick on this.
Scott