I've never understood this either. When the plane starts boarding I stay seated until the line is very short. The flight attendants always find some overhead space somewhere, and everything I'll want is in my personal bag that stays with me.
I assume it's just a psychological thing that first people are more important and they're moving onto the next thing sooner which feels good.
Depends on a number of factors. If I have a carry-on, I'd rather be on sooner to guarantee a spot in the bin for my bag. Generally I've carried on for a reason, and that is not having to mess with baggage claim or in general risk a lost bag.
For Southwest, since seating is not assigned, I'd rather be on sooner so that I at least get the seat I want. For SWA, you get a boarding number and that determines the order you board. Have to check in right when it goes live to have a better shot of getting a number closer to the front of the line.
But any other situation - that is a pre-assigned seat with no carry on, I don't care when I board. And yeah, it's probably ideal to board last.