Message boxes used to have buttons centered horizontally. E.g. here's one you'd get with
MessageBoxW API function on Windows XP:
But more modern systems have message boxes with the buttons on the side, like this same message box opened on Windows 7:
Similar results can be seen with Qt3 vs Qt4+:
Similar pattern holds for message boxes with more than one button. See for example the following "Save changes?" prompts from Notepad on Windows XP and Windows 7:
The message boxes with centered buttons look nicer due to symmetry, so there must have been some good reason for the change.
What is the reason for this switch from centering the buttons to side-justifying them? Do buttons on the side have some important advantage over those in the middle?