One value investing strategy involving real effective exchange rate that i can think of is comparing the currency's REER vs. its spot exchange rate and buying the currency if its spot exchange rate is well below its REER. However, this approach is still quite risky because REER incorporates only trade and inflation data, ignoring many other important factors, such as interest rates for example. Also many currencies, which are "undervalued" according to their REER value are often manipulated (or mismanaged) by their governments.