Motivation

One rarely needs to cycle between three or more keyboard layouts. Usually it's just English and only one of the regional layouts. Therefore, it's very useful to have layout groups, which allow you to switch between a subset of your layouts and change that subset when needed.

Example

Lets say, you've got three layouts - English, German and French. It'd be very useful to place these layouts in a two groups - "English, German" and "English, French". Pressing X11's "Switch keyboard layout" would cycle between English and German by default, but if you press a shortcut to change your layout group, it would instead cycle between English and French.
Due to this, undoubtedly, advaced daemon's feature of allowing each window to have different layouts it'd be possible for user to talk with his German friends in IRC, switching between English and German and at the same time write an email to his French friend, switching between English and French. All that would be required for this is just pressing a shortcut on an email window to have its layout group changed.

Inspiration

Similar feature is already implemented in KDE's keyboard daemon.