Answer:
First of all take a look at the word GEO - earth/PHAGY - eat
Geophagy is the practice of eating earthy substances, such as clay or chalk, practiced as a custom or for various reasons by humans or animals (diet or subsistence). Geophagy can be, as well, a symptom of a mental illness.
Geophagy, the consumption of earth, is common in animals such as birds, reptiles, and mammals. It's reported in rats, ungulates, and primates.
The consume of soils rich in clay, suggests that these animals need clay because of their diet and ecology (the qualities of clay are the stimulus for geophagy).
Hypotheses have been proposed to explain geophagy associated to the consumption of clay.
Warning
What I've written was for information only.
Do not eat the soil, the earth, because it could be toxic, poisonous, infected and, sometimes, radioactive (polluted places). Parasite eggs in the earth could cause worm infestations, for example.
You must contact a doctor before you even drink a suspension of powder of sterilized clay (after a sedimentation) and you must contact a doctor when you need the application of clay on your body. Some body treatments have contraindication. Do not ingest clay without contacting previously a doctor.