Why was the title of Holy Roman emperor given in the first place?

In the year 800, Pope Leo made Charlemagne, also known as Charles I, King of the Franks, the first Emperor of the Holy Roman Empire. Why? It is believed Pope Leo bestowed this title in order to secure loyalty of the most powerful emperor of the region. The Pope also hoped Charles would spread Christianity throughout his realm and perhaps convert the Lombards, Saxons, and other Germanic tribes within the region.

Also, Irene of Athens, a female Emperor, was frowned upon by Pope Leo. Various Popes continued to coronate Holy Roman Emperors up until the 15th century, “in return for protection and independence of the Papacy”.