Why?
It lowers inflation and helps growth, as consumers and producers benefit from lower energy prices, but it has a more direct impact on a number of currencies.
A lower oil prices are a negative for the euro because the euro attracts less reserve diversification from oil producing nations like those in the Middle East.
Slice 10% of the price of oil and Mid-East central banks slice 10% of their EUR/USD GBP/USD buy orders.
A sustained drop in oil will do the US economy much more good than any amount of quantitative ease from the Federal Reserve.
