Question

give scientific reasons(1) it is necessary to excrete waste product​

Answer 1

The waste materials produced during the metabolic activities are toxic to the body and must not be accumulated inside, and therefore it has to be excreted out from the body by the process of excretion.

Answer 2

The waste products like urea etc. are toxic. When these toxic materials are not removed from the body, they get mixed with blood and can damage the cells of the body. Hence it is necessary to remove such poisonous waste materials from our body.