What is the benefit for the JEE advance rank 1 ?

As far as the benefits go, they will only be short term. You will be famous for a while as news networks and newspapers will do their best to make you a household name. Many JEE aspirants will remember you name and use your success as an example. However, once you do go to IIT, nobody would care whether you are first or second. They might ask you what your rank is at first, but that would be the end of it. No faculty would care about your rank. The only benefit for being ranked AIR-1 is that you'll be famous, for a while.