Why not eat flour?

There are many beliefs about food in our daily life. For example, you eat Egyptian but do not eat the same sugar, you eat flour but you do not eat flour.

We can find such differences in foods of the same nature not only in the food mentioned above but also in other foods. For example, light brown rice is considered healthier than white rice. Even brown bread is considered to be healthier than white bread. What is the science behind it?