Suppose Jack is new to MO and he posts a good question. Tim has a lot of reputations in MO and thinks the question deserves more attentions. Why not we allow Tim to offer some of his reputations to start a bounty for Jack's question?
Once a question is posted, we assume that the author wants the question to receive more attentions and good answers. If other people are curious about the answer, in fact the question belongs to them as well.