I booked a room on AirBnB when I visited Los Angleles. I spoke with the owner of the room and he said he made $4000 in 3 months off of his 2 rooms. AirBnB is on to something.
I wouldn't say the calling to the site is based on monetary gain. By comparison, he'd make that same amount if he rented each of those rooms for $670/month (a pretty cheap price for California) - and it'd require a lot less work. I think the bigger call to it is that you can meet new people in an interesting context. It's a fun thing to do, in my opinion.
Having a boarder is VERY different than having people drop in for a night here and there.
He could rent a room for $700 a month or $90/night. The latter works out better assuming he can rent it for 8 nights per month AND allows him to keep in available for friends/family who are coming into town, etc. I think the "fun" aspect of it is definitely part of it for some folks, but I'd wager it's less than you think.