why do I need a pwm and H bridge, after a PID controller to feed a dc motor?
You're guess is correct.  The output of your PID is likely an op-amp or similar circuit capable of driving maybe +/-15V at a few mA.  A motor may require 100V to get to the speed you want and many amperes to get the torque you need.  To do that efficiently you need a PWM switching stage.  Depending on the type of motor you may need commutation logic to keep the field in the right spot too.