Why is Charlemagne remembered in history?

Charlemagne, also called Charles the Great, was a Frankish King who is widely regarded as the father of modern Europe. In addition to being a king, he united much of the territory of (present-day) Europe under the Holy Roman Empire.
In the Middle Ages, it was not uncommon for warlord kings to gain great followings and capture neighboring kingdoms to rule for themselves. When Charlemagne's father, Pippin the Short, died, he split the Frankish kingdom between his two sons- Charlemagne and Carloman. Later, when Carloman died, Charlemagne ignored the rights of his heirs and took over his deceased brother's kingdom as ruler of all Francia. From then on, Charlemagne and his military began expanding and capturing territories belonging to the Saxons, the Gauls, and the Lombards all over the continent. 
Though he had captured so much territory and instituted great administrative reforms all throughout his lands, Charlemagne wasn't officially recognized by the Church until the year 800. Something important to bear in mind is that from the fall of the Roman Empire in the West during the 4th century, the Church and the Crown were separate entities. In the eyes of the people, both were somewhat weakened without the support of the other. In November of 800, Charlemagne traveled to Rome to meet with Pope Leo III, and was crowned Emperor of the Romans. With the Church and Crown reunited, the Holy Roman Empire wielded both secular and religious authority. 
In addition to his administrative reform throughout the Empire, Charlemagne inspired religious reform and implemented academic reform. One of his longest-lasting contributions to Western culture was introducing the Caroline Miniscule- a standardized script for all religious and academic texts to be written in. 
https://www.britannica.com/biography/Charlemagne