Vitamin B12 is important in red blood cell formation, neurological function and DNA synthesis, amongst other vital processes.  Many people are deficient in levels, and as this is not found in vegetables, vegan and vegetarian diets can lead to deficiency also.  Injections every few months can normalise levels, and many people with no known deficiency find there are benefits to the injections such as a boost in energy, alertness, better sleep, and feeling generally better overall.  An exceptionally popular 'boost' every few months for many people.