The currency crises of the 1990s were very detrimental to the internal market. Just remember the big exchange rate crisis we had before the Monetary Union in 1992-93. In Belgium, for example, people were importing Italian cars directly from Italy, where prices were much cheaper. It was possible to do that in the internal market.
Then the car dealers and repair outlets in Belgium started to discriminate against these cars because they were not bought in Belgium. When going to a garage for maintenance, the car was put on a waiting list. In practice, non-tariff barriers were put up in response to the exchange rate effect. And that was jeopardising the very principle of the internal market.