So for the first, I am explicitly describing managers in an agile, team lead style. But, yes. Managers, first and foremost, hire the team. Ideally from feedback, but ultimately the manager is responsible for the team's formation. But then the manager also has to help smooth out bumps along the way. Having someone objective who can help smooth out disagreements can be HUGE. Likewise someone who can give feedback to individuals; a large personality can be a huge asset to the team, but can also leave others feeling alienated; someone who can help carve out space for those others to speak up, and to help coach the person filling the room how to recognize that and also to carve out space for others, is immensely helpful. And maintenance includes progression, both for individuals and the team; devs are often focused on what they're working on, they're not evaluating how to get their work recognized, and more junior ones often aren't evaluating how to grow themselves. It also includes HR issues; having someone who can do the legwork for your Visa sponsorship, for instance, frees you to do real work. A good manager is basically the team's admin assistant as well.
For the second, of course they are. You have to take the two statements I made together. There is a spectrum; at one end are devs working on what they feel is important, but not knowing they're not working on what the business feels is important. At the other is the business deciding everything the devs do, oftentimes changing priorities every week. Having one person as a middleman there can be immensely helpful. That may look like a manager saying "Hey Bob. Jane has this need; work with her to understand and implement it", and handling everything off to a dev, but the point is that the manager, A. Gave Jane (and everyone else) one person to reach out to initially when it came to their needs of the team, B. Helped determine that Jane's need was a priority against all the other needs, and C. Gave Bob permission to ignore all other incoming requests and direct them back to the manager, to focus on Jane's need. That is not to say Bob is not reading and writing; it IS to say that Bob can benefit from someone keeping him from being interrupted by every person in the business with a need who thinks he might help.
For the second, of course they are. You have to take the two statements I made together. There is a spectrum; at one end are devs working on what they feel is important, but not knowing they're not working on what the business feels is important. At the other is the business deciding everything the devs do, oftentimes changing priorities every week. Having one person as a middleman there can be immensely helpful. That may look like a manager saying "Hey Bob. Jane has this need; work with her to understand and implement it", and handling everything off to a dev, but the point is that the manager, A. Gave Jane (and everyone else) one person to reach out to initially when it came to their needs of the team, B. Helped determine that Jane's need was a priority against all the other needs, and C. Gave Bob permission to ignore all other incoming requests and direct them back to the manager, to focus on Jane's need. That is not to say Bob is not reading and writing; it IS to say that Bob can benefit from someone keeping him from being interrupted by every person in the business with a need who thinks he might help.