Theoretically there is no manager in agile. If someone is telling you that they have agile team with a manager they are lying. That mostly mean that they are not ready for some reason to share responsibility across the team - they do not have agile.
Few years ago I was lucky to participate in @jeffpatton agile workshop. This had huge impact on my understanding of application development. Right now I had embraced some agile basic principles and consider them as healthier for internal group dynamics of a team. Agile is literally makes healthier and happier each individual in the team.
People are lazy. Developers are not exclusions. We do not like to work and take responsibility. We easily give out our responsibilities for anyone who will take em. And worst thing you can do is to give all responsibility of project’s success to one person. That what is happening when you put manager in your team.
On other hand people tend to step up and take responsibilities on products they are building and when they do they become more engaged and proud by stuff they do. They stop asking stupid questions and start committing themselves to the product.
We are developers and we are coming to our jobs and spend eight hours in a day to do magic. We really want to build something that makes sense, that could make world better. Isn’t that the best motivation for us? Business, please, spend time to share your passion about product with us!
PS: There is only one case when having a project manager is a good idea. When you have short time project and manager who takes BA role. It just do not make sense to commit in building a team.