What's the reason? I assume this is a US thing?

The stated reasons is patriotism. The real reason is that the military buys a lot of airline tickets and the military member gets to choose from a list of flights from a variety of airlines.

Military also are allowed more than the normal allowance of baggage and carry-on for the same reason.

Families with children are also allowed to board early, because they slow down the boarding process otherwise.

It’s similar to tipping, once one guy starts doing it all the others look like assholes unless they start doing it too. A similar phenomenon happens sometimes in drive-through coffee shops: someone will pre-pay for the coffee of the person behind them in line, then that person is informed that a stranger paid for their coffee. There is then a social expectation to do the same thing for the next person in line to keep the chain of anonymous “charity” going. Nobody wants to be the asshole that breaks the chain. It’s certainly an odd phenomenon, but many people love it.

I suspect that one airline made the first step with that, and it was name-and-shame until everyone fell in line at that point.