We have learned to get park hoppers, as we got more experienced with WDW visits. So many things can happen to make park hoppers useful. First is weather. A downpour at a park might mean early exit. Later, after the shower we're ready to go back. But if it's AK, it's too late to go back. So we go to another park. Then, on another day, we go back to AK to finish what we missed when we left early.
Sometimes Wishes, MSEP and Fantasmic aren't held each night. However with park hoppers, we can plan a park day at a different park, and travel to the park with those special shows. We've also closed out one park, and headed to another park which was having EMH that night.
With park hoppers, we can book an almost-impossible-to-get ADR at a different park from the one we're at that day. For Epcot Flower & Garden or Food & Wine Festivals, we might visit another park in the morning, and head over to Epcot after World Showcase (and the festival) opens at 11am.
And the list goes on.