Blackbeard in particular was all about the aesthetics. He's the archetype for a damn good reason: theatricality makes legends. There's no historical account of him ever directly killing anyone until his final battle, but that's not what the outfit says. He's basically pirate Batman.
And you know what, I'll say it: fashion icon.
I've heard him compared to a rock star. All about the appearance and theatrics.
I loved the depiction of him in Assassin's Creed.
Basically this. Since the captaincy was decided democratically, you don't want to get people hurt or killed - your own or the ship you're boarding. So what do you do? Act terrifying as hell, and make literally everyone you interact with THINK you're going to butcher them into tiny pieces. It helps when you're at least six inches, and more likely a foot taller than most any other sailor (Blackbeard was over 6 feet, most seamen were under 5'8 because of a variety of factors).
Hell why fight when you can just look crazy and scare them into submission.
Drink up me hearties
Yo ho
