Carbs are an essential energy source for the body. This is especially true of the brain, whose preferred source of energy is glucose (a type of simple carbohydrate). Though it's not all bad.

A lot of studies have proved that the above statement is patently wrong. For example:

Both short-term PET and arterio-venous difference studies in humans show that brain glucose consumption decreases as ketone availability to the brain increases. These results suggest that ketones are actually the preferred energy substrate for the brain because they enter the brain in proportion to their plasma concentration irrespective of glucose availability; if the energy needs of the brain are being increasingly met by ketones, glucose uptake decreases accordingly. This decrease in brain glucose uptake when both ketones and glucose are available supports the notion that ketones are the brain’s preferred fuel.

Glucose is actually not a preferred source but convenient. Other cells, like the heart muscle cells, preferred acetoacetate (ketone bodies) over glucose for a stable energy supply.