Best for relief from bloating and associated stomach pain.

Fennel seeds stimulate the appetite (used in anorexia), help digestion and relieve flatulence. Fennel seeds stimulate the production of milk during lactation and have mild diuretic and anti-inflammatory effect. Believed to be of benefit in kidney stones disease. 

Fennel seed oil has been shown to contain more than 87 volatile compounds, including the polyphenol antioxidants rosmarinic acid, chlorogenic acid, quercetin, and apigenin.

Polyphenol antioxidants are potent anti-inflammatory agents that have powerful effects on health.

People who follow diets rich in antioxidants have a lower risk of chronic conditions like heart disease, obesity, cancer, neurological diseases, and type 2 diabetes.

Reference: 

Foeniculum vulgare Mill: A Review of Its Botany, Phytochemistry, Pharmacology, Contemporary Application, and Toxicology - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4137549/