Why do I need a Priest or Priestess? ...any more than I need a Christian clergyman? They're associated with religions, and If I don't follow a religion, why do I need them? What possible use are they to me? What can they do that I cannot?
Many eclectics really follow no real "religion", but rather a spiritual path, which is quite a different thing. I think many, if not most, look within themselves for enlightenment, which is, I have to believe, the only true source.
Many, if not most, also have their own rules and ethical standards.
That being said, for those who practice a religion, a Priest and/or a Priestess is absolutely essential for continuity of a tradition...