I think Hux is fun because he's a good starting point for me to play with themes I like to write, like the big hierarchical structures you mentioned, dealing with highly unstable people, lack of empathy as a main character trait and the very bureaucratic, very banal kind of unspeakable organized evil. The lack of character backstory, which, unlike Rey, probably won't be addressed in canon, is also a plus, because it makes me feel like I have all the room to do what I want with him. And he's got a cool uniform, and a pretty actor to play him.