Stabilising the foot
A ski boot without a properly made footbed is like a house without a foundation. A properly made footbed will add to comfort, control, warmth and stance allignment.
What is a footbed?
As the name suggests it is a bed on which the foot sits. Its main function is to support the entirety of the foot and to minimise movement of the foot inside the boot. If your arch is collapsing or the foot is rolling around inside the boot it won't be comfortable. Also it is inefficient. You must wait until the foot contacts the boot inorder to engage a turn. With a footbed everything happens so much quicker, is more comfortable and because the foot isn't rolling around circulation is vastly improved meaning the boot is warmer.