jack

By this time the boys have really digressed from proper British boys into savagery. Jack uses the head as an offering to the Beast. They hope that the Beast will leave them alone. I suppose the worst part of this is that most of the boys really believe in the Beast and the offering. It is in Jack's interest that the boys believe in the Beast. He rules by fear and, so he says, he can protect them.

did he offer it because he wanted the boys to believe there was a beast if yes why did he do so? was it because he wanted attention?

Like I said, it was in Jack's interest that the boys believe in a Beast. Jack offers them protection from their fears, even if they are not real. This fear of the Beast helps keep Jack in power.

how does the fear help jack to keep in his power?

It is like anything. If you fear someone or something and someone offers you protection, you follow that person.