What is benefit of family over individual therapy?

Depends on the goal. This might be answered by a therapist in an initial meeting. Which therapy is better depends on the issue and goal(s). For example, if the goal is to improve the functioning of the family as a whole or as a unit then family therapy is often better (if family members are agreeable to this). But, if the goal is to improve an individual family member's behavior then individual treatment is better.
Since interactions. between family members impact on the individuals who make up the family unit, I often combine the use of individual and family therapy. When I do, it has to be with families who are open to sharing what they discover in individual sessions with other family members, so the contract regarding confidentiality is different. When it works family sessions enhance individual sessions & visa versa.