If you have baggage, I can understand that you'd want space in the overhead bin. I've had times when there is no overhead bin space above my seat because someone else had taken it.

Having said that, I check in my luggage for all domestic flights so it doesn't usually make much of a difference whether I board early or not. I usually board at the end of my group and board with the last group (if I'm running late).

It would be nice if people who don't clog up the line. One time, as boarding begins, a bunch of people lined up. I was in Group 1 or 2 (can't remember) and I thought, "Wow, this is a long line" and went to the end of the line. Well, the line didn't really move much, which I thought was odd, so I ask the person in front of my if she was boarding with this group. She said she wasn't and was just lining up to board with her group (which was like Group 5 or 6).

I usually don't even get up to board until my Group # is called.