The Fed recently decided not to raise rates because the economy is still recovering. In theory, as unemployment goes down, it should start to increase inflation. However, unemployment is low and inflation is also low. This is probably because people have left the labor force and are no longer counted. Having low interest rates does stimulate the economy but the Fed wants to raise rates, when possible, for several reasons. Policy is supposed to be countercyclical so keeping low interest rates indefinitely could lead to inflation. Also, low interest rates encourage investors to pursue more risky investments for higher yields which can cause bubbles and crashes. Also, staying at low interest rates limits the Fed's ability to use monetary policy at the zero-lower-bound where they cannot stimulate the economy anymore by using conventional monetary policy.