A few points that come to my mind:

• Effort. It's not just some random phpbb board, like ten thousand others
• It's specialized. It's easy to link to perl specific things (like cpan, docs, rt, perl repository, ...)
• The voting system. It encourages useful and polite posts.
• It has an audience/market.
• It has a non-persistent channel for off-topic discussions (CB)
• It's stable (eg the links to 9 year old nodes still work)
• It's fast. (Ok, just kidding)