A currency devaluation of yuan helps countries sell more exports, boosting the economy and help its own manufacturers .Right now the Chinese economy is in the midst of an economic slowdown and has suffered from stock market turmoil, so it can use some extra help.
and Chinese devaluation have affect to other countries such as US exports.
Of course, everything I've just said works in reverse for the United States. As the yuan gets cheaper from the perspective of American consumers, the dollar gets more expensive from the perspective of Chinese consumers. That means it's getting more expensive for Chinese people to import American-made goods, so they're likely to import fewer of them. Lower demand for US goods could mean slightly slower economic growth here in the US. (And the same, of course, is true of other countries whose currencies are gaining value relative to the yuan.)
Tuesday's change presents a dilemma for U.S. policymakers who have often accused China of manipulating the yuan for a trade advantage. A weaker yuan will make Chinese exports more competitive and might trigger complaints from U.S. manufacturers. But the central bank said it is trying to make the exchange-rate system more market-oriented — a step Washington has been pressing Beijing to take.