This explains it better than i can !,

If you are only changing two tyres out of four, whether your vehicle has front- or rear-wheel drive, we recommend fitting the newest tyres on the rear axle. This will give you greater safety in the event of unexpected and difficult situations (emergency braking, tight bends, etc..) especially in the wet. The front tyres generally wear quicker than the rear, especially with front-wheel drive (which applies to the majority of vehicles nowadays). Numerous tests have demonstrated that it is easier to control the front axle than the rear.
This is why KLEBER advise you to fit the new or least worn tyres on the rear axle for :

• better grip when cornering
• greater safety.