Speed multiply  feed rate  change the speed of your print.. and flow rate change the amount of plastic that you put in your print..

so you can do fine tuning (i use it often to slow my print on "difficult" zone of my model... or to speed up on simple large zone..) ..

my kissclicer gcode dont do thing like my slic3r gcode for the same model... so i use also the slider depênding of the flavor of gcode i use...