This of course makes the tool a bit more comfortable to use, since you don't have to figure out the backing block device of your encrypted volume first. But it's actually a security measure too:
On Linux referencing block devices by their device names such as "/dev/sda" is a bit problematic when it comes to security, since these names are reused on unplug/replug. Thus, there's a good chance that various security-relevant operations executed on disks can be tricked, …