I think you're asking :
1 : Identify person
2 : Identify place
3 : Identify weapon
4 : Identify actual murderer
So why play steps 1-3?
Well, I suppose the cheeky answer is 'cause that's the game'
Thematically you're helping the confused ghost unwind the events of a long past crime.
From a rules perspective how you play out the first 3 parts decides if the 4th part even happens, and if so you have some idea of how the ghost provides clues.
Finally your 'score' from phases 1-3 on the clairvoyance determines how many of the cards you can see for the final round which are randomly selected from the 3 (intentionally chosen) cards for the final clue.
1 : Identify person
2 : Identify place
3 : Identify weapon
4 : Identify actual murderer
So why play steps 1-3?
Well, I suppose the cheeky answer is 'cause that's the game'
Thematically you're helping the confused ghost unwind the events of a long past crime.
From a rules perspective how you play out the first 3 parts decides if the 4th part even happens, and if so you have some idea of how the ghost provides clues.
Finally your 'score' from phases 1-3 on the clairvoyance determines how many of the cards you can see for the final round which are randomly selected from the 3 (intentionally chosen) cards for the final clue.