Whenever an application font changes (for example by the user on the system), then this causes all widgets who do not have their own font set to be changed, however, if a font has been explicitly set on the widget, then this font size might not make any sense in relation to the new application one. By being able to set a relative font size, such as 120% of the application font size, this will mean that the widgets that have their own font can also update in response to the new application font.