Regions can be created from containers, but they can also be created from other regions, segments, or disks. Most region managers that support containers create one or more freespace regions to represent the freespace within the container. This function is analogous to the way a segment manager creates a freespace segment to represent unused disk space.
You can create regions because you want the features provided by a certain region manager or because you want the features provided by that region manager. You can also create regions to be compatible with other volume management technologies, such as MD or LVM. For example, if you wanted to make a volume that is compatible with Linux LVM, you would create a region out of a Linux LVM container and then a compatibility volume from that region.