Did Charlemagne get a prestige boost from accepting the crown?  Answer that and the political side of whether he could refuse the crown or not is answered.

One reason can be that Charlemagne saved the Pope ass from the Lombards conquering Italy, Charlemagne was a devout Catholic who maintained a close relationship with the papacy throughout his life. In 772, when Pope Hadrian I was threatened by invaders, Charlemagne rushed to Rome to provide assistance. In 799, the next Pope, Leo III, had been mistreated by the Romans. Leo escaped and fled to Charlemagne at Paderborn, asking him to intervene in Rome and restore him. Charlemagne agreed to travel to Rome and in November 800 and holding a council on December 1. On 23 December Leo III swore an oath of innocence.  At Christmas Day Mass (25 December), when Charlemagne knelt at the altar to pray, the pope crowned him Imperator Romanorum ("Emperor of the Romans") in Saint Peter's Basilica. Doing so, the pope was effectively attempting to transfer the office from Constantinople to Charles. It seems it wasn't without arri?re-pens?e.Many modern scholars suggest that Charlemagne was indeed aware of the coronation; certainly he cannot have missed the bejeweled crown waiting on the altar when he came to pray.