... 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.

TV's Frink said:

Did you watch the movie?

 

Hehe... Only the special effects !

If Ben hadn't sacrificed himself, Luke would have refused to let Han take off without him, and Vader would have fought even harder to keep Ben at bay while the stormtroopers picked off the rest of the rebels.

Basically, he sacrificed himself so they could escape.  Luke never would have left Ben there to die, so dying in front of Luke solves that problem.  Becoming one with the Force so he could help Luke later was just a bonus.

ben knew  he was beaten with vader in front of him and the stormtroopers to the side.    obi wan told vader that if he got struck down he would become even more powerful.  ben did not want to become one with the force but he know if he got struck down he would be a force ghost and could help luke even more by being by his side at all times.

of course if this had been the prequels  ben would had just done  10 flips across the room and   spinned his saber aropund 5 times striking down all the stromtroopers on his way to the falcon.