The price that Apple has had to pay to design a thinner MacBook Pro is to redesign the MagSafe connector of a lifetime, which has now become somewhat longer and less tall than the previous one.
If you want to continue using the old one with the new connector, you have to buy an adapter that Apple has put up for sale in the Apple Store, small in size and with precise functionality, no more, no less.
Personally, it is an accessory that I find of little use logically, but there will be those who have two laptops or an Apple Cinema Display and it will do great.
Source | TUAW