The Proper Scrum Master
A Scrum Master needs to KNOW scrum. They are on the hook to ensure you follow it, to ensure your teams understands why, to ensure road blocks are cleared in an Agile manner, as opposed to taking steps backwards away from Agile and more towards Waterfall. If you're investing in Agile/Scrum, do it right.
Many organizations who implement Scrum just hand the title of Scrum Master off to someone within the team. In some cases this is ok, but remember, a Scrum Master is supposed to know Scrum. As described by the Scrum Guide:
"The Scrum Master is responsible for promoting and supporting Scrum as defined in the Scrum Guide. Scrum Masters do this by helping everyone understand Scrum theory, practices, rules, and values"
Does the person in your team understand Scrum? Do they know why Scrum does things certain ways? What the underlining value each thing brings and why? When asked why by the team, and they will be, can they answer the question?
Handing it to someone with little to no experience in Scrum will not allow that person to help the team understand the theory, practices, rules, and values. Assuming you have great people, they will do their best but remember, Scrum is a different way of thinking. As you un-earth hurdles, and you will, a seasoned Scrum Master will know how to navigate hurdles while still following, and benefiting from the methodology. An unseasoned Scrum Master will do the best they can.
Ultimately, they will lose the faith of the team in them, and in Scrum. They will start to move away from Scrum and tackle problems the "way they always have" driving slowly away from Scrum and back to traditional project management.
Don't just hand out the title to a person on the team. Ensure you give it to, or bring in the right person for the job. Like you would do for any other specialty you hire for. Can't afford it, as I always say, you paying for it anyways. It's a larger commitment than you think. Handing it to a developer you have on your head count takes that person's time away from developing. It will be slow, but as they struggle to know why, and they fight to do the best for their team, it will take more and more time. Ultimately you will be paying for a Scrum Master, who you thought was a developer. A proper person in that role will pay in the long run.
You wouldn't start a restaurant with a manager with no restaurant experience, give your car to a mechanic who read a how to be a mechanic book, or let a web designer do your taxes. The right person for the job will equate to success.