In some countries mail gets delivered to the person whose name is in the letterbox not only to the address. If the person is not listed as living at the address - the mail will not be delivered. C/o field allows delivering mail to a person who lives at the address temporary but the letter will still be delivered to the person whose name is in the letterbox. However by law that person cannot keep the mail if it's c/o-ed to another person. For example in the US "It is a Federal crime and up to 10 years in prison and a $250,000 fine to hide keep or open others peoples mail without permission"