ChainsawAsh said:

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.

Aaaaaaaahhhhhhh....... Oooooooooooohhhh o-Ohhhh.