Why did Charlemagne need to declare the Frankish state an empire? What did it give him?

Due to the fact that almost the entire period of Charlemagne’s reign fell on the conquest and unification of the Germanic tribes in Europe, it was imperative for him to unite this state under one command.
So, by obtaining the status of the Emperor, it was much easier for him to manage these territories and tribes, which had a difference in their culture and religion.
Due to the fact that all of them were now members of one state, they received a common religion and a single ruler.