The liver is a part of the digestive system, but it does much more than simply process food intake.

It is responsible for producing substances that will break down fats and lipids, making foods more digestible.
It also produces several amino acids, which are essential to protein production, without which a body could not function.
The liver produces much of the cholesterol in a body, whether healthy or unhealthy.
Aside from the liver's production abilities, it also acts as a filter. When a harmful substance is taken into the body, the liver is the organ responsible for filtration. For example, the liver filters alcohol out of the blood.
Simply stated, the main function of the liver is to keep an equilibrium in the body. It neutralizes toxins that are harmful, it creates substances that are necessary and it disposes of waste products.