Supplementing with a multivitamin is not needed. It can be beneficial, but the benefits you will see are based on what you're already getting through your diet.
'Multivitamin' is a blanket term to refer to any supplement, usually in a pill or tablet form, that provides essential vitamins or minerals. Not all nutrients are present in each multivitamin, as iron is sometimes omitted. The majority of essential vitamins and minerals are in most multivitamins, in varying dosages.
They are normally used to 'cover all the bases' when it comes to supplementation of vitamins and minerals.
A multivitamin tends to be a good idea if:
You are at risk for several nutrient deficiencies and your diet cannot otherwise be modified
The multivitamin provides adequate dosages to cover the deficiency risk
The multivitamin is a better purchasing option than the nutrients by themselves
Theoretically, multivitamins would provide more overall benefit to people with a lower income, who are financially unable to buy a wide variety of foods. Ironically, this group is the least likely to consume multivitamins.
Multivitamins can also be recommended to pregnant women, due to their increased need for folic acid, which is important for fetal development, and to reduce complications associated with pregnancy in general. Elderly people can also benefit from multivitamins, as they tend to be at a higher risk for nutrient deficiencies, such as vitamin B12.
If you suspect that you are at risk for a nutrient deficiency, you can omit the multivitamin from your daily routine and just supplement the nutrients your diet lacks.
There is insufficient evidence to suggest a multivitamin is associated with less risk of cancer and disease, and this holds true when investigating the most popular compounds in multivitamins, the anti-oxidants.
Some studies note higher mortality associated with multivitamin use, but the relative risk ratios tend to not surpass 2.00 (in which a number greater than 1, or no difference, suggests a stronger possible relation). This is not strong evidence for causation. This weak association is also found when cherry-picking some other studies, but strong relationships between multivitamins and harm have not been found.
Although nutrients in multivitamins may confer benefits when used for a specific purpose, (as some studies note high variability, suggesting some people benefit and others do not) the idea of taking a pill that contains all of the vitamins and minerals to better one's health does not appear to be supported by the literature. However, it does not appear to be significantly harmful either.