Lots of desktop applications and websites add a border at the edges of the application area.
- Some are clearly made for a good reason. For example, a left and right side border on User Experience.SE actually serves as a tab and shows the current area of the website.
Other seem to bring nothing to UX, instead of wasting the space and makes it impossible to use Fitts's law by positioning critical controls on the edges of the window/page. For example, Windows Defender contains a large border, while:
The application is already in a window which has, as any window, a border displayed by the OS itself,
The content is moreover positioned in inner borders.
Is there a reason I ignore which explains the usefulness of those edge borders? Or is it just a designer's mistake?