Engaging in Agile – the more the merrier?

As a project manager and scrum master, I often get the question – why do suppliers sometimes offer a separate engagement (or contract) manager on top of the one or more Agile teams that are asked for in a request for proposal? In Agile environments, teams are self-organized, and the product owner is the only person in the team that makes decisions on scope and delivering the most business value with the available budget. So, why in some cases is such an engagement manager still added to the service offering?