Supplements can help you stay in good health, even if you are generally a very healthy person already. They will ensure you have the right balance of everything you body needs, and if you have too much of something, your body will simply flush it out, so there is no harm in taking vitamins and supplements.
If you are deficient in anything, you will notice your health improving when you start to take supplements. This can be a good idea if you are trying to lose weight, or if you notice that you have brittle nails or hair that is lacking in shine.
It can be a good idea to take supplements if you are trying to become pregnant, for both the man and the woman. This just ensures that you are in good general health and that your body is in the optimum condition to conceive and to cope with pregnancy.