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The original legend was that Jack was a man who was too wicked for Heaven but Hell didn’t want him (or he beat The Devil at a contest which meant The Devil could never claim him).  After he died his soul wandered and he tried to beg to be allowed into Hell (since Heaven wouldn’t have him) but The Devil just laughed at him and The Devil (or one of his demons) threw an eternal ember out for him to light his way.    So Jack carved a lantern out of a turnip (the first Jack-o-lanterns were turnips) to guide his way.

Now the story goes that Jack was a bit of a coward.   And in fact most ghosts are.   So when you carve a jack-o-lantern, from a distance a ghost like Jack mistakes it as another wandering spirit with a lantern, or some kind of demon.   And he wants to avoid others like himself (and demons) at all costs so it drives old wandering Jack away.   And apparently it works on most other goblins and ghouls too, because they’re actually very solitary creatures and they don’t want to mess with a home already inhabited by a goblin.