Quote from Uncle_Ho:
A cheaper dollar makes it more cost effective for other countries to buy our commodities (which are priced in USD i.e. $123.75 crude oil)
If oil is $100 a barrel and the exchange rate goes from 1.5Dollar=1Euro, to 1.6Dollar=1Euro, Europeans can pay over $100 for crude and it is still the same in euros, because they are recieving more dollars for thier same euro.