Both epistemic and instrumental stuff. It's one thing to read about biases in an academic fashion. It's quite another thing to have friends who know you, who also know rationality, who can point out when you're being affected by biases (which you wouldn't have thought to bring up on LW because you were busy being blinded by a bias), who you trust to help you evaluate things with a concern for truth-seeking.

Being surrounded by people who actively work to improve their decision making also inspires me to do better.

And then there's just value in having LW-caliber intellectuals to socialize with, just because there's a kind of enjoyment that comes from that which you don't get from other types of people.