This isn’t unique to metal. The same can be said for bodybuilders, once they go past a certain point. For females it is true for fashion, it is mostly to impress and outdo the other women, most men don’t care.
IMX*, mating success is highly correlated with convincing a potential pelvic affiliate that you find them way more special than their 4 billion co-gendered neighbours, and not so correlated with especially high ability at anything else.
* being a geek, I'm no Wilt, let alone Temüjin...
Acoustic guitars are better for convincing potential pelvic affiliates of their peculiar special-ness. That's a whole different study.
Sick shredding & solos impress other men. Science!
Like my progressive rock band?
"perceptions of playing speed positively predicted intrasexual competitiveness"