A fictional universe can be one story or many. Continuity is an issue either way. Consistency validates and inconsistency invalidates. Effects are meaningless if regardless of cause.
I write fiction. I create imaginary universes. I make the effort to maintain continuity. I do make mistakes. I correct directly, by changing what I wrote, or indirectly, by crafting an explanation in later stories.
Continuity is important whether a story is realistic or fantastic. Realism is dispelled without it. Fantasy is rendered simply ridiculous without it.
TO BE CLEAR: An alternate version of a fictional universe is its own universe, unless the fiction explains the imagined reality as a multiverse.
Consequences matter. A universe without consequences cannot be taken seriously. One with dire consequences can be, if explained logically.
My fiction is usually action-adventure. If it is horror, it is usually as survival horror. The stories are about life and death as an adventure. The heroes live and nameless baddies die. The villains live to muster more goons for the heroes to slaughter.
If I kill a hero or a villain, they stay dead. If they appear in another story, it is in a prequel. To do otherwise is to negate the consequence of death in the fiction. The entire universe loses its edge, becoming too safe to be taken seriously.
An imagined reality must be in the likeness of reality,
whether in its image or not. The make-believe is unbelievable otherwise.