“So what does it do, this … Battlemaster?” she asked.
I thought about that question for a moment. She was trying to understand. Things were still raw on that night; the topic of Warcraft itself was hard to broach. But I had done it, after the celebrations had died down and people got back to the business of farming Fatty Goatsteak. And she had responded.
“Does it give you special abilities? New powers? Make Cynwise … better, somehow?” she continued, trying to clarify her intent. Her familiar eyes met mine.
How do I explain it? The lines of questioning were obviously innocent; her knowledge of Warcraft was practically nonexistent. And it was a good question! What does this achievement, this title, do for you? It’s a diploma of sorts, a badge that you can flash and not say a word, cool and confident as you await the opening of the gates. But it’s not like gear, improving your character’s performance. It’s not like I suddenly had a 5% buff of awesomeness because I’d fulfilled a long list of criteria.
I was quiet too long. The social part of my brain set off alarms letting me know that; thanks, brain. I’m already flailing around with the deceptively insightful nature of this question: what does this achievement actually do for me? Getting it made me a better player, sure – through brute force repetition of actions I wouldn’t normally try. Never again will I take Snowfalls graveyard or race to be the first to recap a base in Arathi Basin. But I will hustle to do those things when they’re needed now, and not hang back, lamenting my role as a ranged damage dealer. So there’s that.
I’m better because of the journey, not because of the diploma.
Say something. Say anything. She’s just looking and you’re thinking about capping flags in Arathi Basin, doofus.
“Mostly, it encourages opponents to pile on and try to beat the shit out of me.”
“Oh. Huh.”
She turned and went back to her work, and I went back to mine.
Cynwise's Warcraft Manual 
There’s a larger story in here, methinks.
Achievements, in general and in my opinion, are something you can use to guide your progression through the game. Battlemaster means you care enough about the WoW PvP game to do all the things required to get it. It’s like asking what a Physics PhD does for you…nothing much really but the process of getting it makes one a Physicist (it’s pretty useless for getting most jobs).
What is the point in running a marathon? What is the point of climbing Mt Everest? Beyond the challenge itself, not a lot.
I take off PVP titles like Battlemaster, Conqueror, or of the Horde when I know I’m in a bad position. If I’m questing while watching TV or at the beginning of a season (or especially if I’m late to start PVPing in a new expansion), I’ll use a PVE title. On the other hand, when someone gets the jump on me while I’m questing but I know I can easily handle it, I’ll switch to one of them mid-fight. In my mind, I imagine them saying, “oh no – a Battlemaster!” In actuality, they probably don’t notice at that point.