[Serious] Why defecation?

[John 6IX Breezy]

Living creatures produce waste because their emissions also introduce chemicals that measurably pollute the environment....

The most immediately concerning waste product is ammonia, which is absurdly toxic...and especially at the levels present with some species.  The idea that this is not waste because, in some sense, it's a natural process, is ludicrous.  Natural processes make cars work, too.  

This is basic restoration ecology, which is why it gets taught in introductory biology textbooks.

Ammonium is an important source of nitrogen, which is subsequently an important building block for amino acids and the synthesis of proteins. Plants need nitrogen, and they make direct and indirect use of ammonium to get it. Ammonium is made available to plants by two processes: Nitrogen-fixating bacteria convert it from the atmosphere while ammonifying bacteria convert it from decomposing matter, and finally, nitrate is produced form ammonium by nitrifying bacteria. Plants then use ammonium directly or convert nitrate back to ammonium to produce amino acids and grow.

In animals the reverse occurs; when proteins and nucleic acids are used for energy, nitrogen is removed in the form of ammonia. Ammonia is, as you mentioned, toxic for animals (not for nitrifying bacteria or plants). It interferes with our oxidative phosphorylation; which is why land animals convert it to urea by combining it with carbon dioxide, aquatic animals dilute it with large amounts of water, and insects synthesize it into uric acid; all facilitating its excretion from the body.

This is all part of the nitrogen cycle: "a series of natural processes by which certain nitrogen containing substances from the air and soil are made useful to living things, are used by them, and are returned to the air and soil" (Campbell et al., 2016, p. 813).

Reference: Campbell, N., Cain, M., Minorsky, P., Reece, J., Urry, L. and Wasserman, S., 2013. Biology. 10th ed. pp.1233-1250.