The WikiAnswers answer is only partially true.
In many cases in the late 1800's and early 1900's, when a European country stopped making silver coins and used silvery-looking base-metal substitutes instead, they wanted to make it very clear and obvious that the new coins were not in fact made of silver. The hole was a clear indicator that shouted "I'm not made of silver".
This is the case with the French coin in question.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis