As a medicinal herb, fennel has proven quite useful. Fennel is helpful in relieving indigestion and digestive problems, bloating, abdominal gas, constipation, colic, heartburn, and irritable bowel syndrome.
Fennel stimulates the digestive system and has a carminative  effects that inhibits the formation of gas. It is also a common ingredient in 'gripe water', a preparation used for treating infant colic. 
Fennel repairs liver damage, and has been used on chemotherapy patients who have sustained liver damage due to chemo. Other claims made as to its potential as a Cancer and Chemo treatment are bogus and poorly researched copy and paste statements more than likely based on an unrelated plant "Nigella sativa" which is sometimes called fennel in other parts of the World. 
The fragrance and taste of Fennel tea has a calming effect on some people which may be placebic in nature as no cause is known. As well, the volatile oils may also possess antiseptic properties.
Recipe for Fennel Tea
2 teaspoons of crushed fennel seeds. The seeds should be freshly crushed, as the oils will dissipate if crushed and stored. Stored seeds are fine, but they should be freshly crushed.
1 cup of thoroughly chopped fennel leaves.
Add the seed and leaves to 2 cups of water and boil on a low heat. Extreme hot temperatures can cause the fennel oils to be chemically altered. They don't become toxic, they simply become useless. Brew the tea for about 2 - 2 1/2 minutes , no longer.
Strain the tea and add enhancements such as honey, sugar, stevia leaves, or even citrus such as lemon zest or even juice.This tea has a mild licorice flavor, it should be consumed while hot and not allowed to sit as much of its medicinal qualities dissipate as it cools.
References - Footnotes
1. Carminative: An agent that prevents or relieves flatulence [ bowel gas and gastrointestinal gases].
2. How To Deal With Liver Damage
3.Flavors of India for Tasty Palates
4. Could this Seed Really be 'Cancer's Worst Nightmare'?