A honeymoon registry is much like a wedding registry.
Even when a wedding registry only lists expensive gifts, you are not under any obligation to purchase one.
Do not put registry information in your wedding invitations -- this is traditionally considered poor wedding registry etiquette and can potentially offend guests.
A wedding registry helps the newlyweds set up their new home together in a comfortable manner.
A honeymoon registry allows wedding guests to help defray the cost of a honeymoon rather than purchasing a gift off the wedding registry.