Private IPs give you more control over your own email reputation. A good email reputation is required in order to have your emails delivered successfully. While we work hard to ensure that all of our shared IPs maintain an excellent reputation, occasionally a user will make a mistake and send something they should not have. This can temporarily hurt the delivery rates of your emails. A private IP insulates you from this risk.
Users who have private IPs are also eligible to have a custom domain set up for links and authentication of their email campaigns.