Oh, some cruises require them. On the Queen Mary II's transatlantic voyages, several nights are designated as "formal," and if you want to dine in one of the restaurants (not the buffets, but your restaurant that you're assigned, that is included in the price of the trip) or go to the casino/theatre/planetarium/most of the bars, formal wear is required. We've made that trip three times--when you have children and a lot of luggage, it's actually less expensive than flying--and each time you're told well in advance that there are formal requirements and which nights are formal, so you can pack appropriately.  I don't have experience with their clothing rentals, because Mr. Mantooth owns his tux and I have several ballgowns/formal evening gowns, but I certainly wouldn't assume they'd have something in my size that I like. (And their definition of "formal" is pretty loose, too; I've seen women on those nights in business-casual type dresses, and no one says anything as long as the rest of the outfit is dressy, too. They just don't want you in denim and tennis shoes; men are expected to wear ties and a jacket, but they don't insist on tuxes, either.) You also have the option of just going to one of the buffets, or having room service, too, if you don't want to dress up. We enjoyed the formal nights but on our first voyage everyone but me was seasick for the first four days or so, sigh, and on the latter ones we usually skipped at least one of them. AFAIK, all Cunard cruises have formal nights like that, but I don't know about other ships (and of course I doubt that, say, Disney cruises, which are all about kids & family, do).