For instance produce, if I can buy lettuce from Poland when the zloty is 0.30 to the dollar why buy from Greece when the Euro is 1.30 to the dollar. I can buy a lot more Polish lettuce with that exchange rate.
However, it doesn't work like that in practice - most European currencies (except the Pound and the Swiss Franc) tend to rise and fall along with the Euro. So while Polish lettuce might be cheaper now, it's due to lower labour costs, not because of being outside the Euro.
Europe misses a strong England to counter weight Germany.
Why does Germany need a counterweight?