Communication is a very important part of Cubigo.
The purpose of 'friends' in Cubigo is to avoid receiving unsolicited messages from people you don't know, or to prevent others from accessing information you don't want them to see.
When someone is your friend, they can, e.g., see your address and date of birth. This information is not visible to Cubigo users who are not your friends.
Conversely, you can deny someone access to all your information by unfriending them. This way you are in control all the time.
The difference with, e.g., a traditional e-mail is that you can select who can send you messages. This way you won't be bothered with unwanted messages (spam).
See also How privacy is controlled in Cubigo.