Which ones are essential?
The eight which are necessary for human life are thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid, and cobalamins. Supplements which contain all these are usually called a super B complex. Often times, a super B complex will contain additional nutrients like vitamin C, too.

What is their purpose?

The B complex vitamins serve a broad array of purposes, but they’re best known for their role in energy production. For example, vitamin B12 benefits the body by aiding in the creation of red blood cells, which are needed for energy transportation and production.

Who is most likely to have a deficiency?
When we don’t get enough through our diet alone, a super B complex supplement is necessary. This is especially important for vegans, because vitamin B12 can only be obtained through meat and dairy. Fortunately, the supplemental form of B12 is made synthetically so it still accommodates that dietary lifestyle. Meanwhile most of the other B vitamins come from vegetables, so if you don’t consume a balanced diet, you may not be getting the recommended amount.

Most people consume adequate amounts of these nutrients through their food. However some of us have problems absorbing it due to abnormal digestive systems, especially the elderly. When this is the case, often times a doctor will recommend a super B complex vitamin or injections to help supplement your body.