The line at the gate to get on the plane

One thing I can't figure out: why do people stand in line in front of the gate for so long to board the plane 20 minutes earlier than others? I don't understand the rush to get *on* the plane unless you have some oversized carry-on that you're hoping to sneak into the overhead compartment and secretly ninja somebody else's bag space.

I can't possibly wait long enough in the airport before I have to get into the small seat (talking economy class here) with a low roof that feels slightly claustrophobic (and I even enjoy flying).