There are a lot of misconceptions and misunderstanding about role of SM. Most of the questions comes from the statement that "Scrum Master has no authority". It is the inherent nature of human beings to "Control" things around them. PMs start connecting that "No authority" means "No control". They feel powerless.
Some of the common functions executed by traditional PM include:
- Assigning tasks to the development team
- Estimating the features and tasks(by sitting with architect or tech leads)
- Resolving conflicts among team members
- status update on behalf of team members with the customer
- Time sheet management
- Resource management
- Change request management
- Single point of contact for the management for anything related to project
- Single point of contact for the on site team for anything related to project
- Leave approval
Responsibilities of Scrum Master include
- Ensuring that everyone follows the scrum rules
- Bringing the scrum team and product owner together
- Removing the impediments from the project
- Helping the team to move towards self organization
- Servant Leader
If you compare the roles of PM with SM, you could see that many tasks are missing in SM. Who would do those tasks ? Everything lies with the team. This is exactly the reason why self organizing team is critical for success of projects practicing Scrum.
It is critical that traditional PM is convinced and has understood the roles of SM properly. Any misunderstanding leads not only failure of the project but also, failure of the entire team in implementing agile. Traditional PMs with half baked knowledge of Scrum Master role would be causing more harm than anything else.