Everyone has had friends of the opposite sex, right? Do you have sex with them? Usually not. However, the idea of a casual relationship is another way of saying "friends with benefits." This is a friendship between two people that allows sexual intercourse. Rather than having a relationship that focuses on the emotional needs, this type of relationship is based only on sexual needs. In between boyfriends? Feeling restless? Horny? Lonely? It's at this time when you call up your your friend and fullfill the "obligations" of a casual relationship. During this, you are fullfulling your sexual needs. There's no need to mess around with with sadness, jealousy, or any other pesky emotions that are found in typical relationships.
In Sex and The City, Carrie is heartbroken over Mr. Big. This is when she calls her friend to fullfill her sexual desires in this casual relationship.