This answer is not applicable to most people, though. Most people are probably worried about flying, about space, and about the whole confusing process and just wait in line because the feel it’s the most likely way to get where they’re going without a mistake.
Note that on United (my main airline), “Premier 1K” fliers like me get to board in “group 1” which is the second set called. United policy allows active-duty military personnel and Global Services passengers (a super-elite) to board first. We also get to pre-book the choicest seats on the plane (if available) including Economy Plus (extra legroom) and the exit row (my choice).
Certainly, overhead space is an issue sometimes and boarding in group 1 means I will have a space for my bag (if I brought one) near my seat. This is handy, and helps avoid the queue on the jet-bridge for gate-checked bags as well as the stress of trying (and maybe failing) to find a spot.
But if you board early even when you check in your bags!
for the simple reason that it’s less stressful if you tend to arrive at the gate near the boarding time, wait in line a few minutes, go to your seat, sit down, and relax. This is much less stressful than hanging around near the gate (or at a club or bar) and trying to get on at the last minute.
As much as people argue that overhead bin space isn’t an issue, it is becoming a bigger and bigger factor, especially as people become more travel-savvy and carry smaller cabin-approved luggage that still needs a fair amount of bin space. Board last, and you’re very unlikely to fit it in, resulting in endless complications and stress. Especially if you’re a frequent or higher-paying traveller and have secured a seat in an emergency row, you have no choice but to stow your luggage in the overhead bin. Only business class passengers tend not to have this problem, but it remains more relaxing to board as quickly as possible and get settled in for the flight. There are various other good reasons that have been well-covered, so it’s easy to fathom this one if you consider the factors.