Vitamin B12, also known as cobalamin, is a water-soluble vitamin. It hosts a variety of symptoms which are very different to each other and can vary from neurological to psychiatric. A severe vitamin B12 deficiency can lead to deep depression, paranoia, delusions, memory loss, incontinence, loss of taste and smell, etc.
Now, it matters a lot because our body cannot make Vitamin B12. The only way to get it is through food or supplements.
Without Vitamin V12, the formation and division of red blood cells can't take place impairing oxygen delivery and reducing a person's energy levels.