From the comparisons I’ve made in the past, they’re also relatively cheap compared to land based vacations. For some reason, it’s cheaper to make your hotel float.
Then there’s places where ships are more inherent to the experience, like transiting the Panama canal, or coastal regions of Alaska or Norway. Places that are too remote to get to by most other means.
But fuck Caribbean cruises. That’s a boat taking you from one tourist outdoor shopping mall to another.