"...but who would Clone Back to the Internal drive?"
A few of many reasons I c an think of, the same reason one would re-image to the original internal hard-drive :
-- Windows or Linux OS blew up beyond quick or normal plow-through-and-fix-it
-- hard-drive only suffered a catastrophic logical failure rather than any physical failure
-- while the OS is basically aok, user profile or profiles were corrupted beyond easy "cobble-copy" repair
-- and so on...