what is the importance of input and output impedance of a RL low pass filter?
I interpret "voltage coupling" to mean minimizing the voltage drop through the impedance of the source.  If the source has 10K output impedance, and your load (the filter plus any load after it) has 20 ohms input impedance, most of the voltage drop--and therefore the power--is consumed by the generator itself, with little being applied to the load.  Similarly, if the load impedance is much higher than the source impedance, nearly the full voltage generated by the source will be seen at the load.