having just been to france, belgium, luxembourg, germany I can tell you being able to use the same currency in all countries is so nice, not ending up with loads of notes and coins from differnt currencies, all too small to exchange and having to calcualte different exchange rates. If you are a business or consumer, now you dont need to worry about exchange rates and you can compare prices easily. Same with everyone using the metric system, imagine you are selling flour say at 1 mark a kg in Germany, while I sell it at 50p a pound in the uk, far easier if we all use the same measuring and currency, coupled with same laws.
On the other hand, I lived in Brussels for 6 months and saw huge amounts of EU waste and double standards...