The Divine Right of Kings
We turn to Alison Weir's book on the medieval queen Eleanor of Aquitaine for the answer. To quote an extract ....
"The ceremony of crowning was established in the reign of Edgar during the 10th century and was based on the rituals used by the Pope to crown the Holy Roman Emperor Charlemagne back in 800 AD."
This conferred sanctity and a form of priesthood on the king. Prior to this the king was styled merely as a lord. This coronation gave him divine authority. Prior to Edward I the regnal years were always dated from the year of the coronation.
The early medieval kings held ceremonial crown wearings at Easter, Whitsuntide and Christmas. The Litany was recited and there would have been feasting and prayer. All this to reinforce the idea of royalty being sacred.