At Airports you can get priority for both process, just fly first or business or have a gold card. It's the same with P&O pay for a suite or have high status..
It's like someone said earlier, buy a basic economy ticket, BA allocate you group 6 , so why think you should board with group 1. However lots do and it causes chaos. Yes there's many "almost" valid excuses why anyone thinks they should be allowed to board earlier , at their leisure.
But in the end things run more smoothly if everyone sticks to allocated time