In a move which has caught some off-guard, the US Federal Reserve has decided to leave interest rates unchanged for the time being.It follows months of anticipation that September would likely be the month the Fed decided to increase interest rates for the first time in 9-years.The Fed said concerns about the strength of the global economy had influenced its decision.Signs of weaker growth and stock market turmoil in China have led to fears among investors about US economic growth.The Fed says its long-term policy is to keep interest rates low until they see a stronger labor market and feel more confident of a return to 2% inflation.So why did the US Fed make such decisions to keep the interest rates unchanged?And what impact does it have on the global market?