The simplest answer to that is variety. Demonic aesthetics often look neat/are fun to draw so having a character with them is nice as a creator and help boost engagement.
As an expanded part of variety, it does depend on the characters' origin and creator's thoughts.
Character wise, it should go without saying, but Hell sucks. That's kind of the most defining feature. And the demonic denizens of Hell are either previously angels or humans. Angels or humans who are being punished for some reason and have a long, long time to think about it and regret their actions. Why wouldn't a human, for instance, who really regrets whatever they did nit try to help others or do better if they had the chance?
Creator wise, being X and having a redemption arc is good character building. A person who is/was in "insert violent profession" but now wants to help others and not harm them? A recovering addict of whatever cleaning up and avoiding the thing they were addicted to? Good chances as a writer for character motivation, world building, and development. Being "intrinsically" evil is just another backstory to work with. And of course, the creator might not be a Bible thumping idiot, and if they even believe in Hell would have it be more an extended sort of Purgatory. Since God is you know, often described as all loving, and a pit of fire and torture for all eternity doesn't jive with the author's interpretation of that. But that's the lovely thing with fiction. Just like zombies, vampires, werewolves, and other fantasy races, the demons can be whatever the author wants.