Zack Snyder's Justice League reveals why the Flash and Aquaman's superpowers require them to wear a superhero costume. The titular superhero team in Justice League Snyder Cut assembles for the first time early on each member's journey, with the exception of Batman and Wonder Woman. By the point the Justice League officially becomes a team in the Snyder Cut, Cyborg is struggling to adapt to the idea of having a robotic body, the Flash still ignores the full potential of the Speed Force, and Arthur Curry Jr. has yet to claim the throne of Atlantis and don his proper Aquaman costume.
Despite their lack of experience as superheroes in the Snyder Cut, the Justice League is a group of extremely powerful metahumans. For instance, Cyborg is capable of destroying the whole world with only a thought and the Flash is able to make small trips back in time, which is why Superman's evil transformation, which submerges the whole world into chaos during the Knightmare timeline, is such a scary display of the Man of Steel's true power. Aquaman and the Flash barely compare to Superman in terms of power in the Justice League Snyder Cut, but they're already so powerful that their clothes can't coexist with their abilities.
Aquaman takes off his shirt every time he prepares to enter the sea. Not only is it a cinematic way to show off his tattoos but also proof that the sheer speed and strength of his swimming would make any piece of human clothing feel uncomfortable. This is why the Atlanteans are used to their own kind of armor, which adapts to their underwater environment. Similarly, Barry Allen's shoes explode as soon as he starts using the Speed Force. What the Flash's suit does is allow Barry to run multiple times the speed of sound without burning off his clothes. Therefore, Aquaman and the Flash get their DCEU costumes out of necessity, with the added value of Atlan's golden armor being an important historical object in Atlantean culture, as shown in James Wan's Aquaman.
The Justice League Snyder Cut and the rest of the DCEU movies do a great job of explaining why superheroes wear their iconic comic-accurate costumes. While Aquaman and the Flash need them for added protection, Batman needs his bat-suit for all the opposite reasons, as it helps him keep up with his superpowered allies. Like Aquaman, Superman and Wonder Woman's costumes are symbols of their culture, as they're both special outfits with unique designs that the Amazons and the Kryptonians kept for very special occasions. Cyborg's jagged body comes from the Mother Boxes, making his robotic vessel a curse rather than a real costume. The Justice League Snyder Cut also gives the heroes in the Knightmare timeline some realistic suits, with the Flash wearing a higher-tech but more dystopian costume and the Joker sporting an apocalyptic look.
Aquaman and the Flash prove that being a superhero has its downsides. Fortunately for them, their costumes add to their image as modern-day gods. Justice League 2 and Justice League 3 would have probably upgraded them with Bruce Wayne's sponsorship in preparation for their final Darkseid face-off. Zack Snyder's Justice League may mark the end of the DCEU SnyderVerse, but future movies will keep revealing that other DC heroes like Hawkman and Supergirl have realistic justifications for why they wear their superhero costumes.
