They're a vacation where everything is taken care of for you. Find a spot, read a book, get all the drinks you want. Need food? Walk over to the chosen food place. Even with thousands of people on board, you can generally find a quiet spot with drinks.
There's all-inclusive resorts, yes, and I've found they're generally more expensive than cruises. If you make your resort hotel float, it's cheaper. I don't know why.
I'd only go anymore if it's a trip that would show things you generally can't see other ways, such as the coast of Alaska or Norway, or going through the Panama Canal. Caribbean cruises are an absolute waste.