As @mb698 have mentioned, every phase is important in project management however, initiating the project, planning on how to divide into segments and executing that plan is the most crucial part of a project. These are an important phase as it will help you understand whether your project will be a success or failure. A project management plan is developed comprehensively of individual plans which includes cost, scope, duration, quality, communication, risk, and research resources. Some of the important activities that help in planning are the development of the schedule, milestone charts, Gantt charts, planning deadline dates and communication with customers based on milestones, deadlines, and important deliveries. Therefore, planning and executing are important.