Why input impedence is important factor for amplifiers
But in all these things what I explained above has nothing to do with
input impedence because Even if we increase input impedence it causes
decrease in voltage gain (for example CE with emitter resistance
amplifier) and hence overall voltage will remain same .So that's why I
don't understand how increasing input impedence is considered good
?although it adds nothing to amplification i.e overall output voltage
remains same (compared to when input impedence is Less) which is main
purpose of amplifier .

What you are missing here is why we use an emitter resistor. Yes, for sure it increases the input impedance and that is a good thing for some types of amplifier but, it is &quot;bad&quot; when amplifying signals from sensors that require an almost zero input impedance for best results. So, for a guitar amplifier you want the input impedance to be between 1 Mohm to 10 Mohm so that you don't attenuate higher frequencies and make the sound dull. For an accelerometer you want the input impedance to be near-zero so that you can convert charge to voltage using a charge amplifier. For an RF circuit, almost always you want maximum power transfer and you make the input (and output impedances) around 50 ohm.
So, going back to the common emitter, if you don't use an emitter resistor you get poor results and these include: -

Heavy distortion on bigger signals
Temperature related quiescent voltage drift
Temperature related gain i.e. gain changes with temperature
Poor frequency response
and of course low input impedance.

So, adding an emitter resistor is something we must do and, if it lowers the gain then so be it; add another gain stage - in fact use an op-amp or other tech.