- On crowded flights, overhead space can disappear quickly, even if everyone sticks to 1 up and 1 beneath the seat. If you travel very light, I can see this not being an issue. For everyone else, you want to get to that space, especially if you are sitting in a row that can't place bags under the seat.
- Some prefer to get on board ASAP so they can start relaxing. For example, get settled into a nice Business seat, the lounge is pretty lousy, or just transferred from an overnight flight (maybe a red eye) and very tired.
- Some prefer to board early if they have a window seat so they reduce the risk of having to inconvenience already seated passengers to get to their seat.
- Families like more time to get settled into their seats and prefer to board early. Not every airline allows families with small children to board first without having the standard priority privileges.
Leaving for an aircraft 5 minutes after Final Call is a risk, especially if the gate is far from the lounge. Yes, it's not very likely you'll be kicked off the flight, but why be a stress on others if you don't have to? There are some airlines and some airports where you definitely do not want to **** around - they will shut boarding and offload you.