Why, where and how we use envelope detector in AM demodulation?
I don't understand how you could have studied AM demodulation and not understand the purpose of an envelope detector. Something is lacking in your studies.  In any case, in AM (the M stands for modulation) all of the information is contained in the  amplitude variations of the carrier.  That amplitude variation is usually called the envelope of the resultant AM signal due to its appearance on an oscilloscope.  Hence, to recover the signal information, the envelope of the AM signal must be extracted.  This is the job of the envelope detector, hence the name.  For analog AM, such as ordinary broadcast radio, an envelope detector is as simple as a diode and a low pass filter. Any communications textbook can provide more information.