As David Deutsch pointed out in “The Fabric of Reality”, factorization of large numbers via quantum computing would harness computation from a very large number of universes. He gave 10^500 as a very rough suggestion of scale.
Absolutely no time lag can be tolerated when dealing with numbers like that. The mechanism for coordinating that number of parallel processors must be instantaneous, or the answer would take far longer to reach than the lifetime of the universe.
This seems like an insoluble problem, but fortunately the mechanism is quantum interference, which is just another name for “spooky action at a distance”.
Which, as we all know, is instantaneous.
So that’s one of its practical uses. I think there are others…