Here’s a better example of the spherical distortion I mentioned. You can see in the subtle bokeh in the background that it swirls in a circular way, where the edges are a little more in focus than the centre.
This means you can separate a central subject more effectively from a background and do so in a unique way. Modern lenses have the focal line run perfectly perpendicular to the camera. In this case the line is curved, running a little further on the edges and a little closer in the middle.