Do people who eat a balanced diet need to take vitamins?

A diet that is well-balanced and rich in whole grains, fruits, and vegetables will provide adequate levels of nutrients for adult humans. In other words, people who eat a balanced diet do not, as a general rule, need to take vitamins.
However, certain categories of people--due to extraneous factors--may benefit from vitamin supplements even if they have a well balanced diet. Several examples follow.
Folic acid, a B vitamin, can be helpful for women of childbearing age who are capable of having children. This can help prevent certain birth defects.
A lack of sunlight can cause a deficiency of vitamin D and persons who have a lifestyle that limits their exposure to natural sunlight may benefit from supplements, even if they have a balanced diet.
Finally, some evidence exists that people over age 50 may not be as capable of absorbing vitamin B12 from food as people under age 50. Persons in this category may also benefit from a vitamin supplement.
That said, while most people who are healthy eaters do not require vitamin supplements, there is no evidence that taking them at recommended levels poses a health risk.
https://ods.od.nih.gov/factsheets/MVMS-HealthProfessional/

https://www.betterhealth.vic.gov.au/health/healthyliving/vitamin-and-mineral-supplements

https://www.health.harvard.edu/womens-health/getting-your-vitamins-and-minerals-through-diet