We like to board ASAP (more hours on-ship). Getting to the terminal early means:
You have first crack at terminal issued stateroom upgrades
You are near the beginning of the line (we were first to board last June...kind of make you feel special as they call your name FIRST)
You get the ressies you want
You are early to request seating time and table changes for the dining rooms, which means the best chances of getting your requests
You can enjoy the pools while they are empty (it won't be like that later in the week)
The picture opportunities are less obstructed with other cruisers
You squeeze a little more time on-ship
You have time to eat lunch at a more leasurely pace (and more than once )
You have more time to unpack before lifeboat muster
A downside is the wait (anticipation) from 10:30 until boarding at noon - but we spend our time well in the terminal.