If you go by the book on Scrum, a Scrum Master can be anyone on a team. They can do their day job (Dev, QA, etc.) and moonlight as the Scrum Master as needed. Maybe this is true for some very high performing teams, maybe not. I have never seen the team that benefits from this approach, I have seen many try it. What usually happens is that the team sees the Scrum Master as a simple secretary, scheduling ceremonies and maybe facilitating them. There ends the value of a Scrum Master. Anyone can do that right?
From my experience this is absolutely incorrect. The Scrum Master should be a dedicated role played by someone with no other responsibility on the team. The Scrum Master idea has evolved in the industry since the book was published and is now vastly more important to a team’s success than running a few ceremonies. In my view and experience, the Scrum Master is in charge of how well everything is running, how well the Product Owner is managing the backlog, customers, stakeholders and interacting with the team. The Scrum Master is responsible for how well the team is doing, each individual member as well as the team as a whole. The Scrum Master is responsible for how well the organization is doing around the team; are they getting the trust and support they need to get a great job done?
For me, the Product Owner is What, Where, When, Who and Why. The team is How and the Scrum Master is How Well. The Scrum Master is coach to the team, the Product Owner and the organization around them. Coaching them on Scrum, coaching them on how to work at the highest levels of adherence to the Agile mindset (values and principals). The Scrum Master role takes on the flavor of the management role in Lean Manufacturing. Coming from the viewpoint of the Toyota Way, there is no other analogous role than the Scrum Master for the role that management plays in that system which birthed many of the ideas of Scrum and still serves as an incredible source of improvement process for it. This is not a role to be taken lightly. There does come a day when a Scrum Master has less to do, even if they are great at their jobs, maybe especially if they are great at their jobs, because if they are great, then their team is great and should need them less than they did in the early days. The organization around the team should be great and so need less from a Scrum Master as well. Even when that day comes, I would not give the role of Scrum Master to someone who has a day job. I would give the job of Scrum Master to a junior Scrum Master and send that awesome one off to work with a team that needs them more or simply give them more teams to work with. Some day I will talk about the career path of a Scrum Master, but not today. Suffice it to say, there is a path for them, and it does not include going back to their day job.