It's part of his personal schtick. Began doing it about a year or so ago. It's his way of connecting directly with the audience at home. He often has a just-you-and-me-talking-here vibe.

Also, presuming that at 1 a.m.-ish when his show is on that much of his audience are insomniac stoners, he'll do bits just for "them" like putting his hand right into the camera and pretending it's a spider. Very entertaining, whether you are hitting a bong or not. =D