Star Wars has hinted at the true purpose of Force Ghosts. The idea of Force Ghosts runs all the way back to the first Star Wars film in 1977, when Darth Vader confronted his former mentor Obi-Wan Kenobi. "If you strike me down," Kenobi famously warned, "I shall become more powerful than you can possibly imagine." He was speaking of a power he had learned over the years, the ability to retain your consciousness after becoming one with the Force.
This ability is associated solely with the light side of the Force, and Matt Stover's novelization of Revenge of the Sith explains why this is the case. Stover worked closely with George Lucas while writing the book, and he was given permission to expand many of the scenes, including a conversation between Qui-Gon's Force Ghost and Master Yoda. "The ultimate goal of the Sith, yet they can never achieve it," Qui-Gon Jinn explained. "It comes only by the release of self, not the exaltation of self. It comes through compassion, not greed. Love is the answer to the darkness." The same idea can be seen in Star Wars: The Clone Wars season 6, which featured a number of episodes in which Yoda learned about this power.
Timothy Zahn's novel Thrawn Ascendancy: Greater Good hints at the true purpose of Force Ghosts. The story is set in the Unknown Regions, and it features a mysterious race who are clearly Force-sensitives. A civil war ravaged their homeworld, and only a handful of them managed to escape the resulting genocide, looking for sanctuary among other peoples in the Unknown Regions. Intriguingly, they are ruled by a woman called the Magys who believes that she can best help her world heal by leading her people in killing themselves so they can become one with "the Beyond."
"You have touched the Beyond, as we, too, will soon rest in it. Do you understand?"
"No," Thalias said. "Will you please explain?"
The Magys did a sort of double twitch of her tongues. Impatience? Resignation? "Our time is ended," she said. "The people are gone. But we may perhaps still bring healing to our world."
"You said that before, that your time is ended," Thalias said. "What does that mean?"
"That there is no reason to go back," the Magys said. "No hope that others of the people still live. So we will therefore die and rest in the Beyond, and through the Beyond bring healing to our world."
"How can you bring healing when the people are gone?"
Another double tongue flick. "Do you not even listen to your own words?" the Magys said scornfully. "You said it yourself: The world is not the people. Our world has been torn and scarred, but perhaps it can be healed. We will join the Beyond and make the attempt."
As Lucasfilm's Dave Filoni recently explained, the Force is affected by all things. "The dark side of the Force is manifested in our greed, desire for power, and fears," he observed. "And the light side of the Force is propagated by selfless action, by living in balance, by overcoming our fears. The Force naturally exists in balance; that balance is thrown out when someone chooses to give in to their fears and then spirals out of control making selfish choice after selfish choice. Fear leads to anger, anger leads to hate, hate leads to suffering." The act of genocide that has been committed on this mystery race's homeworld has tainted it with the dark side of the Force, and the survivors believe their best role would be to become one with the Force and heal it from the Beyond. Naturally, the Chiss cannot understand this perspective, but it is entirely possible it is accurate - and that Force Ghosts thus service the balance of the Force by healing the darkness.
This mysterious Force cult, unnamed in Thrawn Ascendancy: Greater Good, are not the first Force cult to refer to "the Beyond." This concept of the Force appears to be strongly associated with the Unknown Regions, because (as told in Chuck Wendig's Aftermath trilogy) by the time of the sequel trilogy a cult called the Acolytes of the Beyond had gained influence in the wider galaxy. It will be fascinating to see whether this is a connection between this cult and the rest of the Star Wars saga - and the truth will no doubt be revealed as Zahn's trilogy continues.