In my experience on a scrum team, the scrummaster was basically a coach. They helped deal with external issues as the other responder said, so the manager (team leader) didn't waste his time with that and could use his time dealing with team members and doing engineering work too. In my case, the scrummaster had this role for a bunch of parallel teams, not just one, so it was a full-time job for him. The org structure seemed to work very well for the type of work we were doing.