Without them, our bodies wouldn't be able to perform vital tasks such as converting food into energy, building and maintaining bones, teeth, muscle, skin, blood and hair, and keeping our brain, eyes, nervous and immune systems in good working order. (source: Holland & Barrett)
Vitamins have different jobs to help keep the body working properly. Some vitamins help you resist infections and keep your nerves healthy, while others may help your body get energy from food or help your blood clot properly. By following the dietary guidelines, you will get enough of most of these vitamins from food. (source: National Institutes of Health)
Most animals can make their own Vitamin C via a gene called the GULO gene. Some animals have lost this gene over the course of evolution, including humans and other apes, insects, bats, guinea pigs, and some birds and fish.
One school of thought suggests that the reason may be attributed to the fact that humans have included vitamin C-rich foods in our diet for a very long time and that the mutation in vitamin C synthesis related genes have not proven to be a lethal or severe functional defect in most cases. After all, fruits with readily available vitamin C have been part of the human diet for as long as anyone can remember. This led to the mutated genotype being passed on to future generations, resulting in permanent loss of vitamin C biosynthesis in humans. (source: Science Focus)