But that is exactly the point right? If you devalue the common currency then the amount you pay Greek farmers goes "up" effectively, and the price you pay German farmers also goes "up" so the relative costs of things stay the same and nothing changes. Whereas with a translation layer (currency exchange) you can "adjust" what it costs Greece and what it costs Germany to the advantage of one or the other.

A tariff is a way to adjust this economic cost externally which takes it out of the market's hands and put it into a political realm and one step removed from the actual economy that should be adjusting.