There are several substances that are made in the human body, but apparently in lower quantities than necessary.

These include at least choline, creatine and taurine, and also vitamin K2 (which is made in humans by conversion from vitamin K1, which is abundant in vegetables, but with a low efficiency). Vitamin A is also made with a low efficiency from carotene, so a vegan who does not supplement it needs to eat something like a big carrot every day.

It is not known with certainty which are the optimum daily intakes for such substances. All that is known for sure is that their concentration in the blood of vegans (who do not take supplements) is lower than in the blood of non-vegans and that there are studies like this, which suggest that the higher amounts are beneficial.

Because I eat mostly vegan food, I supplement it with these substances. As long as the amount of supplements is small (especially for choline, where both too low and too high daily intakes are known to be harmful), in the worst case they are worthless, but in the best case they improve your health.