Rey harnessing otherworldly power to banish the Sith spirits for good is a much better ending than Palpatine just blowing up because he catches his own lightning to the face, it also explains why Rey would die from whatever she did to destroy Palpatine. The four ghosts also work as a visual representation of the past Jedi literally “standing behind her”, rather than just disembodied voices that encourage her to stand up. And the force ghosts were established from the very beginning as “more powerful than you could possibly imagine”, it’s neat to end to the saga seeing them finally be allowed to directly interfere in the conflict.