A friend with benefits is a friend with whom one has sexual relations. Friends with benefits relationships are often characterized by a lack of romantic involvement and an emphasis on physical intimacy. They can be a way to have casual sex without the emotional attachment that comes with a traditional romantic relationship.
There are many reasons why someone might want to have a friend with benefits. Some people enjoy the physical intimacy without the emotional baggage of a romantic relationship. Others find that it is a way to explore their sexuality in a safe and supportive environment. Still others simply enjoy the companionship and friendship that comes with having a friend with benefits.