... To help Luke in a better way.
1- Looks like O B 1 became more powerful as a "ghost" (rather say a spiritual beeing)
2- Luke would be led to deal with his destiny by himself.
... And I surmise O B 1 had fulfilled his own mission and could leave this world in all tranquility. He took his body with him in other spheres (what Vader hasn't been able to do...)
That's my way to see this.