Why is excretion necessary in living beings?

Living creatures excrete themselves. Excretion is the removal of waste materials arising from normal life processes from the body. It is necessary to eliminate waste products, such as carbon dioxide.

They cause poisoning that slows down critical chemical reactions if they are left to accumulate. The organism rapidly shows signs of poisoning when excretory organs such as kidneys are impaired, and death is rapid unless treated. Excretion should not be confused with egestion, which is the elimination of unused and unwanted material from the body that have passed through the digestive tracts. 

During biochemical processes, some of the by-products are toxic to the cells and body, so in order to maintain life there has to be a necessary to remove those by-products. An example of this is urea (toxic), which is the end-product of the breakdown of amino acids.

Additional Information:

Excretion is an integral process in all types of life. In animals, for example, urine is formed and released through the urethra, which is part of the excretory system. In single-celled species, waste products are discharged directly from the cell surface.

Carbon dioxide and water are utilised by green plants as respiratory products. The carbon dioxide released during respiration is used during photosynthesis in green plants.

During photosynthesis, oxygen is a by-product which is liberated via stomata. Through transpiration and guttation, plants may get rid of excess water. It has been shown that the leaf functions as 'excretophore' and is often used as a means of excreting waste through diffusion, in addition to being a primary photosynthesis organ.