Original Source: Polycount Wiki

When creating the high poly model of your mesh, you will want to make edges more round than they would be in real life. The reason is that as the model gets farther from the camera, the edges will increasingly disappear due to distance, level of detail model swapping, and mip mapping (texture size/resolution reduction). By making the high poly more round, we artificially force the model to retain it's rounder edges for longer and avoid aliasing artifacts longer as well.

Image courtesy of Racer445 "Herbert"