I was going to type a long post but honestly This video for BricsCAD Shape shows it well.
Go to the 1 min mark and you can see how pressing tab lets you select faces which are underneath the one in front.
This would be really useful for when you have groups inside groups. Im currently working on a drawing of a turbine and having to pick my way through the casings, blades and prop shafts etc all tucked inside one another and its a pain to move them. You either have to hide the outer casing or navigate inside the casing to select the groups hidden inside.
It would also help when you hit K and can see the lines being displayed but currently you cant select them.