Names mean something, right? How we address the people we know is often an indication of what we think of them or how we perceive them or our relationship to them. (Mostly. There are times where that doesn’t apply)
In Star Trek: the Next Generation, Captain Picard refers to his crew by their first names when being informal or asking for a favor but also when he sometimes gives orders. This habit switches depending on the severity of the situation and how long they have known one another. In fact, this happens to coincide universally with rank. Riker refers to a junior officer by their name or rank, but they always refer to him as Commander, except for those of equal rank (Doctor Crusher) or those he is intimate with (Diana Troi).
In Harry Potter, Harry is universally referred to as “Potter” by those who do not like him, just as he refers to the professors of Hogwarts by their last names, and Dumbledore refers to the staff by their first names.
While the reader isn’t necessarily given a diagram explaining why certain characters refer to their peers by any given title, the context by which names are spoken and assigned does give great insight into the relationships of those characters. And, if not their direct interactions, at least how they view the person they are talking with.
Just… consider this: The way we address people is a subtle clue to understanding how we think of that person.
Think about your everyday lives? Do you call your boss by their first or last name? Do you refer to your parents as mother and father or mom and dad? If you do, what does this say about the formality with which you perceive your relationships?