There was a further evolution of Obi-Wan that got truncated at the last minute. Notice he went from disembodied voice to hazy and distant image to clear and close image to walking around and sitting on a log next to Luke. Through several versions of the final trilogy's notes and (later, when the last four films had been condensed into one) ROTJ script drafts, during the final confrontation, Obi-Wan stepped back into the physical world to help Luke fight Vader and the Emperor.
Lucas, at the time, ultimately felt that was taking too much away from Luke -- that the final victory needed to be his, both internal and external, and the Obi-Wan bit was written out. But who knows, maybe a re-youthened Obi-Wan was able to pop out to hang with Luke in the new years following that battle. It'd be a fun way to get Ewan in the new films. *chuckle*
But in the moment, I agree it was part becoming one with the Force and part giving Luke focus for the upcoming battle.
--Jonah