I don't know about in other states, but here in MA you need it for things like getting a license, registration for school, etc. They won't accept copies, they want the sealed original.
I have a certified copy, of mine. The original was lost, years ago. We were shuttled around, so much: military, custody battles, etc. I know that Mom has a copy, too. I still have copies of my kids'.
I think mine is in my Dad's safe deposit box. Mom always said she would give them to us, but it is probably safer there anyway. We lose Earl's birth certificate until we move again. Then we put it in a "safe" place where we won't lose it. LOL Then we lose it again until we move again. Never ending battle.
My father threw out the old biscuit tin which held all the family's important documents (birth certificates, passports, baptismal certificates etc etc), but we let him off as he was very ill.
I now keep the copy of my birth certificate with my passport in a kitchen drawer, though I probably should move it somewhere safer. You can get copies for about $20ish in the UK, and they only take a week or so to come through.
I have my original birth certificate - I had to have my mother find it when I applied for my passport (which was no easy task) and kept it since. It's safer with me anyhow.
Speaking of birth certificates, does anyone know where I can get a 'birth certificate' for my furbabies? Like those human baby ones, where you fill in the details.