I'm not sure I understand the "Reward Existing Answer" bounty. Specifically, why aren't new answers eligible for the bounty? Why would someone start a bounty for an existing answer?
This is for users who come across an answer they think is great that they want to award some of their reputation to as a bonus. Maybe there's an answer with a lot fewer upvotes than another that you think deserves some recognition or just to encourage a poster to continue creating quality content.