Star Wars: The Last Jedi destroyed Kylo Ren's (Adam Driver) sinister mask in an effort to get him to step out of his grandfather's shadow, but for some reason Star Wars: The Rise of Skywalker felt the need to bring it back. The sequel trilogy had the monumental task of establishing several new legacy characters to carry on the Star Wars mantle, including former Stormtrooper Finn (John Boyega), brash and arrogant fighter pilot Poe Dameron (Oscar Isaac), and of course, lonely-scavenger-turned-Jedi Rey (Daisy Ridley). However, the most consistently compelling new character to come out of the sequel trilogy was Kylo Ren, the Dark Side acolyte manipulated by Supreme Leader Snoke (Andy Serkis) into enforcing the iron will of the First Order. The character deeply resonated with fans due to his unique inner turmoil and redefinition of the traditional confines of villainy.
Audiences first discovered in Star Wars: The Force Awakens that Kylo Ren is actually Ben Solo, the son of Han Solo (Harrison Ford) and Princess Leia (Carrie Fisher), corrupted to the Dark Side and stolen out from under them. This is heavily elaborated on in The Last Jedi, where it's clarified that Ben turned to the Dark Side after being betrayed by his mentor Luke Skywalker (Mark Hamill), an event which left Luke jaded and bitter at his own failings. In turn, Kylo sought the monstrous legacy of his grandfather and tried to embrace the Dark Side, despite the inner turmoil within himself.
One of the most striking things about Kylo's first appearance in The Force Awakens was his design, which gave off an atmosphere of mysteriousness and sinister intent due to his mask. The Last Jedi takes Kylo's mask away from him and in the process, attempts to make a comment on his conflicting nature as a character, but that commentary is undercut by The Rise of Skywalker's pointless decision to give the mask back to him.
In The Force Awakens, Ben is desperately trying to extinguish the light that is naturally within him. He clings to his version of what Darth Vader's legacy is and wears an outfit that resembles him in an attempt to conjure up the same gravitas that Vader had. It's even revealed in the comic book Star Wars: The Rise of Kylo Ren #3 that the idea for the mask was given to him by one of the Knights of Ren, who wore one in order to be more intimidating. Kylo's mask represents a childlike attempt at playing evil without the commitment to it, and it isn't until he kills his father that he truly embraces the corruption presented by the Dark Side. Subsequently, when Rey scars his face in their first battle at the end of The Force Awakens, it's a physical representation of the scar left on his soul by the murder of his father.
The Last Jedi builds off of this by having Kylo return to wearing his mask at the beginning of the movie, to hide the increasingly tumultuous moral conflict at the core of his being. Supreme Leader Snoke sees right through this and berates him for it, demanding that he "take that ridiculous thing off." Snoke wants Kylo to expose his scar to the world and air out his own conflicted feelings about the murder of his father. He does this by destroying his mask, and over the course of the movie Kylo steps out from under Snoke's rule and does what his grandfather never could: usurps power from the Supreme Leader and takes it for himself. By abandoning the mask, Kylo discards his childlike emulation of Vader and commits to forging his own path, one informed by the duality within himself.
Shockingly, these developments are immediately reversed in The Rise of Skywalker, as Kylo rebuilds his mask at the beginning of the movie. Not only does it taint the character development made in The Last Jedi, but the mask being rebuilt doesn't really serve a purpose considering he only wears it a couple of times in the film. Abrams' decision to bring back Kylo's mask highlights the fundamental differences between the two films and their intentions regarding Kylo's character: The Last Jedi sought to explore the moral dichotomy driving his actions, while Star Wars: The Rise of Skywalker simply made aesthetic choices for the sake of homage and fan-service.