What use is vitamin B in the human body?

The B vitamins function as part of co-enzymes. A co-enzyme is a small molecule that combines with an enzyme and activates it.

The B vitamins are "helpers". The energy yielding nutrients--carbohydrate, fat and protein--give the body fuel for energy; the B vitamins help the body use that fuel. Vitamin B6 helps the body use amino acids to synthesize proteins; the body then puts the protein to work in many ways--to build new tissues, to make hormones, to fight infections, or to serve as fuel for energy.

Vitamin B12 and folate help cells to multiply, which is especially important to cells with short lifespans that must replace themselves rapidly. In short, each and every B vitamin is involved, directly or indirectly, in energy metabolism.