We need bitcoin because
1. It is not backed / controlled / manipulated by any centralized entity.
2. It is feasible to do a trade between two parties anonymously
3.One world, One currency.
4.A cryptocurrency purpose is to enable someone to become their own bank.
5.Internet brought freedom to knowledge while Bitcoin will bring freedom to financial world.
6.There are 2 billion people in the world who don't have access to a bank account. Banks don't serve them because the infrastructure is too expensive and it's not profitable to build operations there. Bitcoin solves that. You have a bank on a $20 cell phone. The 2 billion unbanked population needs Bitcoin.