In the strictest sense, the introduction of a ghost to lead the characters and the play to their final conclusion is illogical, since in respect of realism, ghost don't normally wrap up family events, even tragical ones, for us. Yet this question has to also encompass the author's aims and ambitions with the work. Since a first draft of the flashback explaining Jacob's and Ben's past relationship was composed to be told from a ghost's (Jacob's) point of view, it is clear that part of Cook's intent, aim and ambition was to combine realism with what some critics have identified as symbolism:
While the dramaturgy of Jacob’s Wake may represent, in some views, a step backwards to “contemporary realism” (Parker 35), or simply a failed attempt to fuse realism and symbolism (40), [it has] excited ... much critical
reaction ... ("Stranger Figuration and Outsider Depiction in Newfoundland Drama, 1945-")
In this sense, then, the inclusion of a ghost is a logical stylistic choice. Further logic unfolds when considering that Cook describes Jacob's Wake as an attempt at a classic tragedy in modern times. Tragedies, like Macbeth and Hamlet, have traditionally included the visitation of ghosts (and a Hamlet effect may be what Cook was trying for in the above mentioned rejected first draft).
after the failure of a ghost device in early drafts, [Cook] settled for framing monologues at the beginning and end of the play which direct the spectator’s gaze through Ben. ... describing events he now feels were pivotal in creating the breach with his father .... ("Stranger Figuration and Outsider Depiction in Newfoundland Drama, 1945-")