Why was  King Alfred too important to be called “the Great” ? 
What did he do ? 

Alfred the Great  is the only English monarch to be accorded the epithet the Great.  He reigned Wessex   (one of the seven Anglo-Saxon kingdoms ) in the period going from 849 to 899 AD. 
The title "great" was given to him because of his great leadership over the land and the people he served. This great leadership was shown most through his resistance to the Danes ( Vikings ) as they invaded the north-west ern of England trying to conquer the Anglo-Saxons in the 800's. 

King Alfred's education as a child was important to his reign. He did not have a formal education and learned to read and write only after he became king in 871, but he did receive great instruction throughout his life. At the age of four, Alfred went to see Pope Leo IV in Rome for instruction. The Pope later adopted King Alfred as his spiritual son. Most of the Pope's instruction to Alfred was concentrated on Christianity and not the liberal arts. This instruction was well suited for his future role as he would spend most of his reign defending the Christian Anglo-Saxons.
In the year 871, Alfred undertook the government of the Wessex. 
After taking over the land, King Alfred faced with some military choices. He was already somewhat experienced in battle because he had led several campaigns against the invading Danes. Alfred did not want to fight, so he kept the peace by paying tribute to the Danes for four years. 
The next year, 876, when the Danes came to collect their tribute, they did not leave the land. Instead they stayed and lived off of Alfred's people for the next two years. By 878 Alfred was fed up with supporting the Danes in his land, so he attacked them and won, forcing them to comply to his terms and pay tribute to him. The Danes, however, still had power in many other places. In 886 Alfred decided to attack London, a Danish-held city. He hoped to diminish the lands ruled under the Dane law since he thought of them as heathens. He succeeded in capturing it and making the West Saxons quite powerful. It is said that at this point Alfred earned the title of "King of England" because of his great success. In late 892, the Danes decided to make a violent return. Alfred was taken off guard, but, with a new tactic at sea, was able to defeat the Danes once again.
Although Alfred's greatest achievement during his reign was the defeat of the Danes, he also had other accomplishments. He pushed for better education and helped make learning important in the lives of the people of his land. Alfred built schools and cathedrals. 
This was necessary during his reign because education had declined due to the fact that the Danes were looting the monasteries and churches which were the centre of education. Alfred believed that learning "makes life more rewarding and enjoyable;...the worst thing of all is ignorance" (Alfred University). He also kept the Anglo-Saxon Chronicle  and established a code of law based on the teachings of the Bible. This helped maintain social order.
King Alfred the Great died on October 26, 899 and was buried in the Old Minster at Winchester.