Flat metal panels are subject to oil-canning, among other problems. Any distortion may cause ripples to show up. It can also cause a vibration due to airflow so

Flat metal panels are subject to oil-canning, among other problems. Any distortion may cause ripples to show up. It can also cause a vibration due to airflow sometimes. Making the panels out of thicker metal help both of these problems, but also increase weight. Using a curved panel gives you a little margin to move small problems to the top or bottom of the panel. A curve next to a curve doesn't give a strong visual cue for lateral offset, but a sharp angle next to a sharp angle does. This is also why a very slight orange peel texture in paint is not always a bad thing.