* Being "cloud-ish" and have a pay-as-you-go system * Computing a price to use a 256MB of RAM for 3 hours in real currency was too complicated (like, for example : 0.0000174€) * Having no surprise at the end of the month, as some hosting providers compute the global use and bill the consumption at the end of the month (you may have surprise) * Using credits, to create a new server, adding a new network interface, etc, you do not need to buy 'resources' like it was before * There are two interfaces : historic usage of credits by resources, display the credits consumption as what you use in real currency * No multiple expiration date by product/resource * ...
Many of those advantages are described here : http://wiki.gandi.net/en/iaas/references/billing/credits
As usual, any enhancement/improvement is welcome :-)