They're useful for large families who want to stay in the same room, and for families of any size looking for more space. Family cabin pricing varies widely, depending on the size of the room and how many people will be sharing it. (Cruise line fares are per person, not per room like hotels.)
Can kids stay in their own room on Carnival?
If the relative or guardian insists on booking separate staterooms, minors must either be directly across the hall or next door. Guests 12 and under may not be assigned to a balcony stateroom without a relative or guardian (25 years of age or older) traveling in the balcony stateroom with them.