Why government intervene in the foreign exchange rate market?

The primary reason why any gov't would intervene in the FX markets is to effect their monetary policy directly onto the value of their currency against other currencies (the exchange rate). The reasons why a gov't want to do this are plenty, but the usual reasons are:

1) Affect the current price of goods domestically.

2) Encourageimportsor exports, depending on the goods.

3) Stimulate the local economy

4) Stimulate the economy of a friendly emerging nation.

There are times where just the news of a change is enough to effect a significant outcome in the exchange rate. But other times, direct buying/selling or trading of currency notes(or currency equivalent like bonds and otherpromissorynotes)