As Harvard Health Publishing clearly states:

Despite their iconic status, there is no evidence that multivitamins enhance health and well-being or prevent illness. … Supplements for prevention? It's a disappointing scorecard.

In an area filled with snake oil, I require a higher threshold for belief. I need to understand how something works. The same article summarizes:

Most people stand to benefit from vitamin D, many from fiber, and some from fish oil.

so I take vitamin D and fiber supplements.

Vitamin D

Vitamin D helps you absorb calcium for healthy bones. To produce vitamin D naturally, we need sun exposure. However, since I work in an office and wear sunscreen, and I don't often eat the important sources of vitamin D, I take the supplement.

Fiber

On top of promoting “regularity” (lol), fiber reduces the risk of cardiovascular disease. I should eat more whole grains, fruits, vegetables, nuts, and seeds; I'm trying! Until then, I take psyllium husk fiber in pill form.