It is necessary when using cream products. Creamy products like liquid foundations, liquid or cream concealers and cream blush can fade or be weighed down by gravity. This is the reason why makeup looks do not remain in their original glory throughout the day.
A translucent powder, or setting powder, will keep it in place making sure it lasts longer and the logic behind its creation.
By absorbing the oils that begin to creep up on the T-zone area, it allows makeup to stay put longer. Dusting it over your eyelids will reduce the oil ensuring that your eye makeup look lasts.
If you have not made it a part of your makeup routine, do so and see the difference it makes.