So... a bit more research revealed some very good information already here...
...and that's why they're called Weejunsa brief history of penny loafers
Seems there are a couple of theories as to why a coin was inserted into the shoe. Some say it was used to make a call home in an emergency. Others say that it was a way of indicating the dating status of someone. A coin in your shoe meant you were committed.
Anyway, I've learned something new today. I usually do on SF.