Those who want to export stuff keep their currencies down. Like China does.
But if you want to buy, you have to keep your currency value up.
Problem comes if you want to sell but currency you sell in is high valued (unless it is something essential which everyone just has to buy from you, like oil).
And even worse issue appears, if your currency is worth peanuts bu your do not sell but BUY stuff in huge amounts. This is the problem USA now has.
For everything USA buys, it has to shovel increasing amount of dollars. Benefit for exports are not enough to counter this, as USA has become system which exports rather little, most industries are aimed at internal consumption, but imports insane amount of stuff.
This internally turned production is also what has left many US industries highly wasteful. Like steel industry in Bush regime which needed support to learn to be efficient when facing competition. And now US car industry is suffering from it's own years of ignorance as their products are losing their attraction.