I remember my Primate Evolution class and the mention that Gorillas chew bark and spit it out. The hypothesis, according to my professor, was (if I remember correctly) possibly they do this for some nutrient they cannot get elsewhere.
As it turns out, new research has shown that decayed wood provides over 95% of the gorilla's dietary sodium.
Sodium is important for the healthy functioning of living organisms, and is involved in muscle contractions, regulating blood pressure and maintaining water and acid-base balance, among other things.
Read more in this Livescience article.