The liver is the second largest organ in the body. (The skin is the largest) Its sheer size is indicative of how important it is to maintaining a healthy body.
The liver is the primary cleansing organ in the body. It filters the blood to remove toxins. It never stops working, but can slow down and become sluggish due to over work. When that happens, you become toxic and much more prone to disease.
Think of the liver as a worker in an office sitting at a desk. He has an inbox and an outbox. The inbox contains the toxins you have ingested. The outbox is the toxins he has processed and is sending to the urinary system to be eliminated. The more toxins you send in, the harder he has to work. The harder he works, the more tired he becomes, and the less he can process. Imagine that inbox overflowing. He can't keep up. He begins to file the work away so that once he catches up on things entering his inbox, he can concentrate on the toxins he has stored. If he never gets a break and continues to store the toxins he cannot process, things begin to break down. He gets sick. You get sick.
If you cleanse the liver, it works more efficiently. There is no more storing of toxins. There is no more struggling to catch up. It works the way it should, and you stay healthier.