How might currency control measures affect the trade of a certain nation?

When you impose such policies, you declare how much of a certain currency can enter your country, or can leave your country. If you have different currencies this could harm your economy because it might prevent others from trading with you due to currency differences. If you do things like Europeans, then you can introduce a new policy that abolishes your old currency and adopts a widely used one like the Euro. This might boost your economy because others might invest.