You get to visit many locations without repacking. And you don't have to do any of the driving once you are on the ship.
You get to go out to dinner (and lunch and breakfast) every night without having to drive there, or decide where to go, or how much to pay/tip, etc. Plus, if you don't like your order, get something else!
You can let everyone in your group roam the ship without worrying that they will get lost - they can only go so far! So you can tell the kids to get "lost" for a bit, and feel comfortable that you won't lose them.
There is always something to do for everyone. Shows, games, tours, classes, gambling, drinking, swimming, sleeping, working out, dancing, singing, eating, etc.
The view of the ocean is better than ANY view you will have from a hotel. And the view changes from port to port.
Consistently good food - sometimes great food.
Excursions - opportunities to do things you could never do at home.
Just a few thoughts.
RCCL - Adventure OTS (Honeymoon!)
CCL - Carnival Miracle
RCCL - Enchantment OTS