If you have a balcony, the ship's robe is great to wear out there in the morning while you enjoy a cup of coffee and watch the ocean go by. It's also handy to throw on and greet your cabin steward or room service at your door, or to wear to spa treatments.
Bathrobes are not considered appropriate attire at any time in the dining rooms, optional restaurants, bars or show lounges. However, if you show up in your robe at the lido buffet to grab a snack, you probably won't raise eyebrows.
If you're on a ship that provides use of a bathrobe as a reward for being a repeat passenger or for paying top dollar for your cabin, it's best to keep that robe in the cabin unless you don't plan to remove it. The ship may say you can wear your robe to the pool, but if you leave such a coveted cruising status symbol on a chaise while you take a dip in the pool, it's liable to disappear and you'll be on the hook to pay for it.
Don't confuse "complimentary use of a bathrobe" with a "complimentary bathrobe" you may take home as a souvenir, or you'll find the price of it on the credit card you used for your onboard account. Cabin stewards keep tabs on the robe assigned to your cabin, and if it's not left behind when you disembark, you'll pay for it.
Robes are washed and reused from cruise to cruise, and are usually one-size-fits-all, although some lines may have some super-sized robes available if you request one. If your robe gets soiled during the cruise, it's OK to ask your cabin steward to replace it with a fresh one.
If you fall in love with your robe, you may buy it, although you'll be given a brand-new one to take home. For example, Royal Caribbean offers their logo-emblazoned "Luxurious Lounge Wear" in three quality levels, from the $19.95 thin cotton-polyester blend to a $50 terry model, all the way to an $80 full-length terry waffle robe with velour collar and cuffs that will occupy half your suitcase going home. The robes also have matching slippers. Carnival's robes may be purchased for $49.
