Why Soft Skills are Crucial for Project Managers
In successful projects, project and line managers are likely to have shared authority. The project manager will negotiate the line managers' commitment to the project and works with them, not around them. The project manager probably provides recommendations regarding employee performance.
One of the most difficult aspects of your position as a Project Manager relates to the simple reality that in most organizations it's unlikely that project managers will have direct control or formal authority over the people on the project team. This is fundamentally different from leading a department, where the manager ordinarily exercises formal control and there is a hierarchical relationship between the supervisor and subordinate. Project leaders lack formal authority, and therefore must rely on influence and persuasion to gain cooperation from the team members.
You'll have to get things done through the people on your team without having any direct control over them. Among your most valued tools will be the ability to persuade and influence, as you seek to form a group of diverse personalities into a unified team with commonalty of purpose.
Reference: Fast Forward MBA in Negotiating and Deal Making (2015), Roy J. Lewicki and Alexander Hiam.