I don't know why in-person group is important, but it seems to be - all the people who have replied to me so far saying they get useful rational practice out of the LW community said the growth came through attending local meetups (example). We can easily invent some evolutionary psychology story for this, but it doesn't matter why, at this point it's enough to just know.

No need for evopsych, it's a matter of bandwidth and lag. When you're meeting someone face-to-face, they can see your gestures, hear the inflection and emphasis in your voice... that's a lot of additional data, compared to a flat transcript of even the most carefully-chosen words. On top of that, you can notice when they're getting distracted and compensate in realtime.