For people eating a balanced diet with fruit, vegetables, whole grains, protein and dairy or alternatives, a routine multivitamin offers little proven benefit and does not replace a good diet. Large studies have generally not shown that multivitamins prevent heart disease or extend life in well-nourished adults.
Multivitamins can be genuinely useful in some situations: restrictive or low-calorie diets, vegan diets, malabsorption conditions, after some weight-loss surgeries, older adults with poor appetite, and during pregnancy or when trying to conceive (where a specific prenatal with folic acid is recommended). Targeted single supplements often make more sense than a broad multivitamin when a specific gap is known.
More is not better. Fat-soluble vitamins such as A, D, E and K can build up to harmful levels. Talk to a doctor or pharmacist before starting, especially if you take other medicines or have a health condition, and tell them about any supplements at appointments.