We can break down and absorb many of the other nutrients from plant matter, like vitamins. On top of that, undigestable cellulose counts as fiber, which is also good for us.

The trouble is that stalks, leaves, and other plant parts contain a complex molecule called cellulose. It's a tough molecule to break down. In fact, our bodies can't even digest it. Most people think of termites as pests because of the damage that they do to homes and other structures. But termites harbor more than 100 species of bacteria in their guts. The stomachs of termites contain bacteria that can break down cellulose. (Agricultural Research Service, U.S. Department of Agriculture). These microbes digest cellulose and other complex molecules in wood. Without their bacteria, termites wouldn't be able to survive on their woody diet. The dark depths of a cow's stomach are home to cellulose-munching microbes as well, says Paul Weimer. He's a research scientist with the U.S. Department of Agriculture Dairy Forage Center in Madison, Wis. "Cows are natural processors," Weimer says. "They make their living by eating plants, and bacteria carry out their fiber digestion." Weimer says that the bacteria in a cow's stomach produce many different enzymes that break down the cellulose in grass and other plants in a cow's diet. These bacteria hold cellulose-digesting enzymes on their cell surfaces in a structure called a cellulosome. What's more, the bacteria attach themselves to cellulose fibers in the cow's stomach and digest them on the spot. http://www.sciencenewsforkids.org/articles/20060412/Feature1.asp Cellulose is produced in enormous quantities by plants and degraded by fungi and bacteria. Some animals such as cows and termites have symbiotic microbes in their intestines that can digest cellulose. What would happen if you ate nothing but grass and hay? You would probably starve. But cows eat grass and hay almost exclusively, yet they grow big and fat. Why is this? Because cows, like other ruminant animals, have a special type of stomach called a rumen, which is home to billions of microbes which can eat grass and hay. These bacteria, fungi and protists provide nutrients that the cow can digest. Without these microbes, the cow would die. Rumen microbes help the cow eat hay, which is made of cellulose and other polymers, which are long molecules that the animal cannot digest, but microbes can. The microbes break down the cellulose into smaller bits which the cow can take in, or absorb. The microbes use special proteins called enzymes to break down cellulose into small bits. The rumen is home to billions and billions of microbes, including bacteria, protists, fungi, and viruses. These many different rumen microbes form a complex community of organisms that interact with one another, helping the animal digest its food. http://commtechlab.msu.edu/sites/dlc-me/zoo/zacmain.html The Russians have done research on this topic some decades ago. In order to digest cellulose, an enzime is needed to split it. In general, almost no animal (the human is no exception), neither bark beetles nor wood borers produce such an enzime. The cellulose-munching microbes eat the cellulose and thereby gain weight and multiply. The vegetable protiens get converted to microbial protiens in their bodies which are readily digested in the stomachs and interstines of anilmals (and humans). The glucose, amino acids, and fatty acids produced in the bodies of microbes are absorbed by blood without any treatment. [B.F. Sergeev, Physiology for everyone, Mir Publisher, Moscow, 1973, pp. 53 - 58]

If you have a Vita-Mix machine you can break the cellulose down to the cellular level releasing 98% of the nutrients available in the food as opposed to 25% of the nutrients we are able to get from masticating the food with our teeth.

We chew the food thereby breaking through the cellulose layer so that the digestive system can get to the nutrients inside. And the cellulose is fiber which also aids in digestion.