Why Is The Archbishop Of Canterbury So Important In The Church Of England?

The Archbishop of Canterbury is actually a Primate of the Church of England,  which means that he has huge power within the Church.  The Archbishop of York is also a Primate, but he is a Primate of England, whereas the Archbishop of Canterbury is a Primate of ALL England.
Canterbury is a particularly Holy site within the Church of England.  The title of Archbishop of Canterbury first emerged in the seventh century.  This was after St Augustine first landed in Kent.    He was ordered by Pope Gregory 1 to come to England to convert the pagans and in AD 597 he landed on the Isle of Thanet in Kent, where he set about converting people to Christianity.
The Pope then ordered 12 suffragan bishops be appointed.  Augustine then appointed himself as the Archbishop (chief bishop) of Canterbury.  So Canterbury became the place where Christianity was effectively born in England and the Archbishop is its leader in today's Church.

He is the number two in the Church, under the Queen, and, therefore, as the most senior priest literally controls the Church in much the same way as the Pope contrtols his Church.