Why is the Vatican a place of pilgrimage?

While religious sites are assumed to be the objective of a pilgrimage it can also be used in other conotations. A pilgrimage assumes a great moral search, or at least the pilgrim is taking the journey to enlighten themselves or out of a sense of reverence. You can make a pilgrimage to "Strawberry Fields", "Graceland", "DeLay Plaza" or the "Vietnam Memorial Wall". It doesn't have to be to Jerusalem or Mecca or the Vatican.