The optional reason (or custom message) a user can enter for why they are adding a bounty to a question.
Sometimes you have no idea why someone would post a bounty on a question. For those times, should they feel the want, a user can leave a hint or note as to their motivation in slapping one around.
Could be they want to drive more attention. Or the current crop circle of answers just aren't up to snuff. Maybe there's a stellar answer already and they want to juice up the user who posted it some more for their efforts.
Who knows? With bounty remarks, now you can too.