In the inner ear are these tiny little hairs, and over time you lose these hairs, they are made to protect the ear drum and loud noises, from a concert, or factory, over time can harm these hairs, possibly leading to permanent hearing loss.
Wearing ear plugs at a concert works because you will still hear the sound, and you wont be harming your hearing. I know this because i've worked in factories most of my life, and my hearing is okay, but not what it used to be.
Unfortunatly, permanent hearing loss is just that, permanent. Once you blow out your ear drum I believe that is it, you will be permanently deaf, but I think with hearing loss, i'm not sure but I would presume a hearing aid works with progressive hearing loss, so the less loud sounds entering your ears is a definite plus.