As a raid leader, you should have the ability to organize and optimize the resource in the team. You should focus on the goal and do not deviate from it. You should be familiar with the theory and practical operation involved in maximizing gear choices and spell selections. Now and then you need to do some maths to calculate the impact on my gameplay from warlock spell mechanics and other factors. Similarly important is make a good preparation for the foundation of a good raiding.
Without foundations, you would never get a house. Being able to carry out an excellent, DPS-maximizing spell rotation on a target dummy doesn't necessarily mean a good preparation for coping with a more complicated situation. Making clear the tactics for a fight can make you a solid foundation for a beautiful victory. Remember, you should try to be a smart raider and not a passive robot.