Updated to indicate the DM vastly prefers Official Clergy to be 'earned in game.'
Obviously, certain faiths such as Siamorphe, who values the status of one's birth over the actual talent of the character, have reasons to apply on creation. However, titled clergy is earned in game, more or less.
There may be other situations where you're character say- converted to Cyric while remaining in the church of Shar or Bane and uses their resources to fuck over said Faith…