I think part of problem here is that there's a notion of a generic right answer to these questions and that typically the "right answers" are those that get passed down from the enterprise world.
I've worked at companies with no managers and it's worked fine. At one place I worked we basically just had a sales teams, an engineering team and a CEO. There were no managers. The sales people raised tickets for bugs and requests for features that would make their lives easier, these would then get a glance over and approval by either a lead dev or the CEO and someone would pick up the ticket. As engineers we would also raise tickets when we identified areas of the system we could improve.
This worked really well and it was an extremely efficient way to work for us but what we gained in efficiency we lacked in accountability and scalability. But when you're a company with less than 50 employees neither matters too much.
For large enterprises and public companies there's clearly far more value in having managers and various other non-engineers roles. For one as an engineer I literally don't want to be responsible for any big non-technical decisions in large companies which provide critical products and services. But in larger companies there's just far more value in having a clear hierarchy of accountability.
That said, I do think a lot of small companies think they need managers when they don't. And I think the reason they believe they need these roles is just because that's how it's done in the larger corps.
To some extent this might be a matter of personal preferences too. I tend to like working independently and quite like being given fuzzy requirements. I know I've worked with some devs who prefer working with well defined requirements and get uncomfortable when making decisions that haven't been formally approved. Depending on your engineers and culture the value of managers might vary.