Why somebody need Bullet Proof hosting?

bulletproof hosting is good if you are hosting adult sites, or doing things that aren't illegal, but may be high-maintenance such as an IRC server.

It's good for hosting sites that are subject to problems that need a higher level of support.

Lots of spammers like these hosts too. All of the major backbones in the USA will turn you off if you're caught spamming, so many of the providers who wouldn't necessarily care whether you're spamming or not still won't let you on the network because your activity could get them shut off.
"bullet proof" hosts are the ones who stem off of foreign backbones that don't give a crap whether their mail servers are blocked or not.