Those who had dragons used them in three main ways.
First, dragons were deployed to support troops on the battlefield — “close air support,” in military parlance — half the time. For example, in the Battle of the Goldroad (Season 7, Episode 4), better known as the “Loot Train” battle, Daenerys rides Drogon to destroy the Lannister forces that sacked Highgarden. But they’re not used alone; they’re air support for Dothraki riders, Daenerys’s mounted cavalry.
Second, dragons were used in strategic bombing campaigns against castles or cities to achieve some political goal nearly 38 percent of the time. For instance, as described in “The World of Fire & Ice,” Aegon the Conqueror used the dragon Balerion to burn Harrenhal and kill his adversary King Harren during the Conquest.
Third, in about 13 percent of cases, dragons were used in air-to-air combat against opposing dragons — primarily in the Dance of the Dragons, a Westerosian civil war about 175 years before the events depicted in the TV show.
Nuclear weapons, by contrast, primarily serve as a deterrent.