B12 is mainly found in animal products, so vegans may need to consider its supplementation. Non only vegans though, because scientific literature shows that meat eaters may also have low blood concentration of B12.
B12 may support mental function, cardiovascular health and it is important in energy production.
I will only supplement with B12 alone if your blood tests show you are deficient. Otherwise chose a vitamin Bs complex, as they all work synergistically together.