Comment 7 for bug 1500282

Yes, this scale (also editable by GDK_SCALE for debugging purposes) are intended to handle hi-res displays like retina. The DPI scale is just one part of it - the application is supposed to also provide hi-res graphics (pictures, icons, controls) for those modes.

IIUC This is also a reason why it's integer scale - to easily scale bitmap images like icons or to provide hi-res icon set.