i want to know the usage of a low pass filter and the usage of the phase
Low-Pass filter:  Your ADSL line filter so the higher frequency "broadband data" does not impact voice



Likewise from a power electronics point of view &amp; a PWM inverter, there maybe a need to "smooth out" the higher frequency PWM to produce a sinus voltage 

High-Pass filter: These are used in the loud speakers to reduce the low level noise.

Likewise as Andy aka stated: hifi crossover

 

Use of calculating phase... depends on the field of interest. Control stability is one, group delay is another. 
You tagged this "power electronics" do you have something specific in mind...