Managers are not needed but management is. There are many ways to fulfill the task of management, one way is to have a manager, one way is to have the developers handle it themselves.
If you're lucky you can get a team of devs who are capable of self-managing and communicating with the outside world and handling everything else they need to do to be successful besides just writing code (writing JDs, talking to customers, dealing with budget, etc.)
But hiring a team like that is hard. Developers like that cost a lot of money. The law of comparative advantage is a real thing. If you can't hire a team that can manage everything themselves you can at least piece together people who can fulfill the different functions.
And that's how you end up with developers and managers.
If you're lucky you can get a team of devs who are capable of self-managing and communicating with the outside world and handling everything else they need to do to be successful besides just writing code (writing JDs, talking to customers, dealing with budget, etc.)
But hiring a team like that is hard. Developers like that cost a lot of money. The law of comparative advantage is a real thing. If you can't hire a team that can manage everything themselves you can at least piece together people who can fulfill the different functions.
And that's how you end up with developers and managers.