Cruise ships, by nature, are surrounded by salt water and salt air just about all the time. If you've ever lived in a beach area, you know what that can do to your car. Ships are also made of steel, and are subject to the same sort of deterioration and weather-blasting your car can take. The interior areas of a ship are under constant wear and tear as well. Carpets, plumbing, air conditioning and every public area on a vessel is in constant use year-round. When you get off a ship the final morning of your cruise, the onboard personnel have just a couple of hours to clean, press, change and shine every inch of that ship before a new set of passengers starts to embark that afternoon!
The cruise lines will take each of their ships, generally once a year, into what's known as a drydock or wetdock situation for a couple of weeks. Drydock means they actually pull the vessel out of the water in a giant hangar-type area, where everything is steam-cleaned, scraped and repainted. Carpeting, bed linens, and any broken or damaged items on board are replaced or repaired. Engines and internal mechanisms are updated, repaired, replaced, and revamped.
Wetdock is the same procedure to a lesser degree, where the ship remains in the water while being serviced. Obviously you can't do as much in a wetdock scenario as a complete drydocking, where the underside and propellers are exposed. Sometimes the cruise line may take an area of the ship, such as an obsolete bar, and turn it into a library or something similar. Maybe a former disco becomes a caviar bar. Times and priorities change.