I don't want that default to be visible to anybody but me. I just want to set who gets the bounty when it expires if I don't change my mind. Currently the system picks a winner if you don't pick one based on some criteria that makes sense, but can also result in the wrong person getting it.
Bounties are basically an auction. You are offering up a certain number of points to award a response. You want to award those points to the best response (offer) before the time expires.
So, given that they are an auction, I would like a way to nominate the highest bidder at any given point in time. This saves me from having to go back just before the bounty expires and picking the best answer available. I can then change who I've nominated at any time before the bounty expires.