Sometimes, when using a GtkFontSelection, commonly through either a font button or a font selection dialog, you cannot or do not want the user to select a style or a size.

For example, you only need a font name and you cannot make actual use of a style or size, thus for the user being able to select either is only confusing.

Or you have a different interface for other font features and it doesn't make sense or not work well to select them in the button or dialog.