Disclaimer: Even though fairy tails may or may not exist, every part of Fairy Tail that does belongs to Hiro Mashima.
Badass
They weren't just for looking badass.
The piercings provided him with an important defense mechanism. He was always equipped with sufficient metal to shield his skin from attack, wrapping it around his forearm or stretching is across his entire person.
And the protection they afforded wasn't just physical. He hadn't really considered it at the time, but his body modifications created a solid barrier between his heart and those who might harm it. He looked scary. Even without the piercings, he was a huge man, a tower of rippling muscle. He needed to be physically strong to control such an unyielding element. Fire and air were much more malleable than metal and required less brute force to bend. His face was all sharp edges and shadowy hollows, like a fairy tale forest in which the heroine gets lost and raped in metaphor. But before the piercings, it had at least been soft, the same delicate texture as everyone else's and therefore, able to be hurt as well as to hurt others. One had to approach him before they noticed the red eyes, the faint dimples and ridges that indicated a clenched jaw, and the perfunctory quality in the set of his scowl, indicating that it was there to stay.
It was amazing how sticking a few pieces of metal in your face was as good as putting up a "Trespassers Will Be Shot" sign on a front porch. Hardly anybody knocked on the door to Gajeel's inner sanctum anymore. So he didn't have to think about what would happen if he let them in. Didn't have to consider that they would walk out once they'd said their piece and never return. Didn't have to fuss over cleaning up the absolute mess that cluttered his corners, festering in darkness. Sometimes people threw pebbles at his windows- whispers they thought he couldn't hear. But they never came close enough to look inside. It's true that eyes are the windows to the soul and he got used to people looking at his eyes instead of into them, seeing nothing but red, the blinds firmly closed.
And then there was Levy. Even when she was terrified that a true monster probably crouched in wait in the shack of Gajeel, she watched him, curiosity pouring out of her eyes like the light of ten crystal chandeliers shining into the street at dusk. She looked for that thin sliver of light where his blinds met the wall, for some gap to peer in though. She knew there had to be something back there, something that he was trying painfully hard to hide. She was curious. She couldn't leave a puzzle unsolved. And even though Gajeel didn't seem like a puzzle at first, his actions matching his appearance perfectly, she didn't know which had come first- the image or the persona.
Because those piercings sure made him look badass. And Levy had read more than enough books to know what that meant and to feel an itch beneath the tingles of fear.