I looked around for the same answer and found a reasonably well written answer below. Tldr; being that the only real difference is if your neighbours recognize your title - and the implied pecking order of your title. If you for example declare yourself Emperor in a neighbourhood of Kings you have implied yourself superior, and you may have to go to war for the title to be recognized.

https://history.stackexchange.com/questions/52754/why-did-the-person-in-charge-of-a-principality-not-just-declare-themself-king