Mixing in weaker coins with the mark to get the euro makes the euro weaker than the mark was. That helps German exports to non-euro countries.
Within the euro, the link is even stronger. Before the Euro, there was an agreement to keep exchange rates more or less fixed, but the fixed rates would get adjusted when they grew too unrealistic. When that happened, the Italian lire, Greek drachme, Spanish peseta, etc. would get devalued, making German products more expensive for people in Italy, Spain, Greece, etc. That mechanism is gone now.
Within the euro, the link is even stronger. Before the Euro, there was an agreement to keep exchange rates more or less fixed, but the fixed rates would get adjusted when they grew too unrealistic. When that happened, the Italian lire, Greek drachme, Spanish peseta, etc. would get devalued, making German products more expensive for people in Italy, Spain, Greece, etc. That mechanism is gone now.