They are posing in all the artworks and loading screens.
To look [Menacing].
They’re trying to attract mates.
Have you ever posed?
It Feels AMAZING
Can you imagine Anduin gettting stabbed by the Heart of Azeroth?
For your entertainment.
Why suddenly Anduin getting stabbed by the Heart of Azeroth? 
Your Heart of Azeroth has an unused essense slot!
What the hell is this

It’s a joke anyway
Yeah I get that part lol 
Everything has a beginning. Even posers.
Ginyu Force tryouts.
No the real reason is WoW is a cartoony franchise, and strong silhouettes and poses are a hallmark of recognizable cartoon and comic characters. Think about how strong protagonists like Superman always have their chests thrust out heroically while villains tend to be slouchy, shadowy or have otherwise more introverted body language.
That pose isn’t for you. It’s for “The Ladies”
You just answered your own question.