All the food we eat is made up of various nutrients, many of which are needed daily by our bodies. The basic building blocks of food are:
Proteins
Carbohydrates
Fats
These are needed in roughly balanced proportions. In addition, there are certain minerals - such as iron - that we need in very small quantities,and other nutrients called vitamins, which are relatively recent discoveries and which we also require. At different stages of life we have different needs for these; if we are under stress, or sick in any way, our requirements of some may be higher. Each individual is unique so one person may have a higher need for some nutrients than another.
Some people take multi vitamin/mineral supplements as a safeguard, to ensure that they have at least the recommended minimum of each nutrient. If they are good quality tablets, without too many fillers, this may be useful for some people. However, if we listen to our bodies, and generally eat healthfully, we should be able to meet most of our requirements, most of the time, from our food. When individual nutrients are needed in excess - such as Vitamin C to help fight an infection - it's then better to take that on its own rather than as part of a general supplement.
The main problem with junk food - such as sweets - is that they contain almost no useful nutrients. And since our modern lifestyle is fairly sedentary, it can be hard to burn off excess calories if we also eat sufficient good food to gain all that we need. If we lack certain nutrients, we may seem fine for months, even years; but will gradually fall into ill health which may not have an obvious cause.