Why are French shops asking customers to round up the price of their purchases?

The next time you are shopping in France you might be asked 'l'arrondi solidare" and the question you are being asked is whether you want to round up the total of your purchases to the nearest Euro. 
The reason you are being asked is that the extra money goes to selected charities. Big name brands in France are taking part. 
Last year 259 charities benefited from the donations.