Why is Hentai Censored in Japan?

I remember reading something about it months ago, but never fully understood.
Who is it censored for? Will there ever be a day where this might change?

@13... What?

When a hentai is made in japan, its not made censored already. They edit it and sell it, when its brought to other countrys, they simply sell the original without the censor. BTW its an old law from the Meiji era... never got changed...