Shock value is used in more places than just rock music, although Eminem, Marilyn Manson, etc., certainly made careers out of it. Rush Limbaugh uses it, Andy Kaufman used it, even Socrates used it to an extent, I think.
Shock value is used for one of two reasons:
- You can't think of anything else to do, or
- You want to get someone's attention very, very fast.
Everyone has exploited shock value at one time or another in their youth -- maybe something simple, like a tattoo, or a piercing. Sometimes a bit more extreme -- like, say, taking up satanism.
Can't find a creative spin on shoot 'em ups? Release something like Postal. Want to get contraversy for a talk show? Make fun of women's rights. The dollar doesn't care why it was spent -- a Marilyn Manson CD costs you $15, whether you're a fan of it or if you're exploring the nature of the beast for your informative article in the Church paper.
The good news is that shock value itself runs out after about a year or so. The Onion put this very eloquently in their article, "Marilyn Manson Now Going Door to Door Trying to Shock People." | 
