The groupings on the D-Z color scale are just convenient categories useful for communicating basic visual information to the consumer. Colorless (DEF) are diamonds that will not appear to have body color to the majority of observers in real life. Near colorless (GHIJ) will not show obvious body color, particularly face up, to most observers, but those will high color acuity will be able to make distinctions. The bigger range implies that if you are one with color sensitivity, you are best suited to the upper grades in near colorless. If not, you may be perfectly happy with lower near colorless. For people who prefer some warmth, the tinted portion of the color scale is where to focus.
The thing to remember about diamond vs CZ is that the optical properties of diamond tend to do a much better job of reflecting the colors and shadings of the physical environment back to the eye of the observer. They also tend to produce more colored sparkles (fire). Diamonds may therefore not appear as colorless as other transparent materials in some physical and lighting environments, despite having no body color.
Type IIa diamonds are those with negligible amounts of nitrogen impurities in the carbon lattice at the atomic level. Nitrogen impurities are responsible for varying amounts of yellow color in diamond. Some assume that type IIa diamonds must be therefore be more “colorless” than type Ia diamonds. But other impurities or distortions in a diamond at the atomic level can also impact apparent color.
The only practical benefit of type IIa is the added rarity, as the vast majority of diamonds in the normal range (D-Z) are Ia. From a market perspective, the only demand for IIa is with a subset of shoppers for D FL or D IF diamonds who seek the rarest of the rare. If a diamond has some detectable body color in the lab, e.g E or F, then the question of diamond type is largely moot.
As Garry mentioned, when GIA released the color grading scale the market already contained an ad hoc collection of rating systems that were generally ABC or A, AA, AAA type designations. Wanting to avoid any confusion with those systems, GIA decided to start the scale at D.