To get more genuine interaction on your blog. The more people interacting on your blogs the better the payout is likely to be for you. Also if you follow the steem-bounty account then you'll be able to find out about all of the active bounties that are going on. It means that you are likely to go and read, upvote and interact on a blog that you might otherwise not have seen. Of course there is an element of risk and reward for the person offering the bounty - you might not get back the amount you've put down as the bounty. In that sense it's no different from buying votes in order to bump a blog up the trending, hot or promoted pages but at least this way there is more of an incentive to contribute something meaningful to the discussion rather than just upvoting for the sake of making a bit of sbd. I'm sure you are right, there will be some scammers or spammers who look to abuse it but plenty of others who will benefit in the right way.