"supposedly" before Yoda leaves the Jedi temple for the last time, he tells Obi Wan that he must retrieve this journal as it details an ancient Jedi secret of becoming one with the force in order to "survive" after death. I am totally paraphrasing, but this would explain why both Yoda and Obi Wan appear as apparitions in the original trilogy, but countless others do not...
Ironically, in a similar theme, Palpatine tells Anakin about a sith lord who also had this death defying ability, which in the trailer , Anakin responds with"can you learn this ability" and Palps says "not from a jedi"....and later, at the opera scene, Palps goes into more detail, naming the sith lord who mastered this power (or knew of it, anyway)....5 bucks if (in the next 4 minutes) you can tell me the sith lords name....
Before retiring, I took the liberty of "searching" this topic, and while there are mentions of The Journal Of The Whills, and even an old thread, they are not relevent to Ep3. Relatively insignificant to some, perhaps, but not redundant. Having somehow allied myself to this thread, I intend to sleep in flame retardant undies! 😇