In a perfect world, there would be no need for managers. Everyone would come together as a collective, agree on what needed to be done, and then go off and do it. People would always follow through on their commitments, or report issues immediately so that the group could adjust plans. There would also be no need to document and communicate a plan (like the barn-raising scene in Witness, the canonical example). People would understand their role, and how it contributes to the whole, and would always act congruently with the goals of the group.
Of course, we don't live in that perfect world. In the real world, we need managers at various times, to help make the group more effective. So, with respect to the title of your post, yes, managers are necessary. The secondary question that I believe you touch on, "does manager have to be a dedicated role", is more complicated. In the company I work for, many of our employees move in and out of management roles, in addition to their day-to-day project responsibilities. I've seen other organizations that are successful with a similar approach. I think that where the team consists of highly trained professionals, who have the requisite maturity, it is possible to operate with a part-time manager.
Of course, we don't live in that perfect world. In the real world, we need managers at various times, to help make the group more effective. So, with respect to the title of your post, yes, managers are necessary. The secondary question that I believe you touch on, "does manager have to be a dedicated role", is more complicated. In the company I work for, many of our employees move in and out of management roles, in addition to their day-to-day project responsibilities. I've seen other organizations that are successful with a similar approach. I think that where the team consists of highly trained professionals, who have the requisite maturity, it is possible to operate with a part-time manager.