Combat in 4e Dungeons & Dragons is a tactical affair. The effective use of conditions and the importance of movement demand that an effective party do more than just attack. The party needs to attack as an efficient cohesive unit, where every resource is used to best advantage.
This requires that someone call the shots. A default assumption might be for the leader in the party to be that person, after all leaders lead. But is that really the best decision? Is the leader dazing, stunning, slowing, moving or immobilizing their opponents?
Didn’t think so, no that duty rest primarily with the party’s controller.