As a good source of magnesium, basil promotes blood flow. As a good source of Vitamin A it promotes healthy eyesight, and because of eugenol (a major component of the oil produced by basil's leaves) it has also been shown to be an effective anti-inflammatory in the same vein as aspirin and ibuprofen.
Basil is an excellent source of vitamin K, manganese, iron, vitamin A, and vitamin C. It's also a good source of calcium, magnesium, and omega-3 fatty acids.
Basil isn't simply for internal use. When basil's oils are extracted to make an essential oil, it is used for treating cuts, wounds, and skin infections. It's also been shown to fight acne.