The distinctive "aspect" it provides. In addition to adding a more pleasant bokeh blur to background objects near the edges of the frame, it also allows objects on the periphery that are in the same plane as the subject when that plane is perpendicular to the optical axis of the lens to blur as good, although not to the same extent as the background elements.
Some people want it, especially in portrait lenses where the edges are usually intentionally blurred. Some people, such as those who perform the reproduction of flat documents or macro work, do not want it and prefer a flat field. You pay your money and make your choice.
Here is a typical image linked to Flickr. In all honesty, the curvature of the field in front of the flat field would not matter much with it because there is nothing near the edge of the frame that is in the plane plane perpendicular to the optical axis of the lens at the distance of the subject. Here is another one. See how the parts of the bench facing the model but closer to the edge of the field of view are as sharp as the focus point, while the parts of your arms that are the same distance as your camera face are not everything so sharp? Also this one (where the flowers in the same flat plane as the focus point in the middle but near the edge of the frame are more blurred) and this one (in which the elements at the edges are not as sharp as the elements in the center , although all are in the same plane that is parallel to the camera sensor).
Note that in this, even raindrops that are the same distance from the camera as the subject are blurred. In this case, the railing in the lower left part that is closer than the main subject is focused and shows the shape of the frontal focal plane that is a part of a sphere.
On the contrary, observe how the EF 100 mm f / 2.8 L IS macro maintains a flat focus field to the edge of the frame. It is demonstrated in this case by the tree along the left edge. The hair of the subjects on the right in this shows the flat field. With this, the flat field of the EF 100mm f / 2.8 L IS Macro made the shoulder in the lower left part of the frame as sharp as the subjects' faces. If a lens with field curvature had been used, such as the EF 85 mm f / 1.2 L II, the shoulder would have softened slightly and perhaps less distract the subject's face. The same could be said of this.
Another advantage of using a lens with field curvature is that there is less error in focusing distance when using a & # 39; focus and recomposition technique & # 39; if the camera rotates around the optical center of the lens. Most of the error usually introduced with & # 39; focus and recompose & # 39; It is because the center of rotation is in the center of the photographer instead of the center of the lens. But even when the camera rotates around the optical center of the lens, there is more error with a lens that has a flatter focal plane than one with a field curvature that matches the focal length of the lens.
Update: the two photos included in this question are perfect examples of the types of shots for which an uncorrected field curvature lens would be useful.