Louis XIV of France was quite an enthusiast, sometimes undergoing as many as four a day - often during meetings with dignitaries. Mayan shamans are believed to have used hallucinogenic enemas. Made with mead, tobacco juice, mushrooms, and morning glory seeds, they must have induced majorly altered states of consciousness. Proponents of enemas today argue that they clean away microorganisms, impacted feces, and dead cells, while relieving constipation and helping backache, fatigue, headache, loss of concentration, and other maladies. Quite a list.
Colonic irrigation involves flushing the entire colon with running water.
The Gerson therapy you mention uses coffee enemas, plus a special juice diet, as a treatment for cancer. Its purpose is to detoxify the body and restore normal function to poisoned cells, which then mobilize to fight tumors. Coffee by this route is stimulating and addictive, claims of enthusiasts notwithstanding. Whether coffee enemas really detoxify the liver or any other organs is an open question. I don't think there's any health need for enemas and colonics, despite their interesting history.