It could be used for some public email which receive lots of email from real people e.g. support email. The bidding is used to help identify important emails, if you have an really important email, you can place more bid to attract the receiver, also it helps to prevent spam(like in Facebook you have to pay $100 to send an message to Mark Zuckerberg)

Also you can white list some tag (e.g. Gmail support email+tag@gmail.com), so if email send to an specific tagged address will automatically passed through, so you can use this address for newsletter etc.