What are the benefits for strong economies like Germany and France for being members of the European Union?

Recently I was reading things on the European debt crisis. It seems to me that when there is a financial crisis, strong economies in EU, like Germany and France, are supposed to help the countries in trouble, like Greece, out of the crisis, with debt or whatever. But this seems to be an economic burden for them being part of EU.

I wonder what the benefits for strong economies like Germany and France are, for being members of the European Union. Please note that my question focuses on the positive side for them to stay in EU, namely I am not really asking what political troubles Germany and France will get into for leaving EU.