Question

why do we need to take out the average atomic mass of isotopes of an element for computation ?

Answer 1

There is no element without isotopes , The avg.is taken because we do not know which isotope takes place in the reaction we do now.
This is the main reason for writing isotopes of any element with it's starting letter.Eg, HYDROGEN HAS 3 ISOTOPES NAMELY PROTIUM, DETURIUM, TRITIUM.