[indigochill]

I see a good manager like a good interface. It's probably unnecessary on a small project, but on a large project you'll be thankful for that piece of the machine that keeps the signal-to-noise ratio high and transparently lets you do the things that need to be done.

For example, I develop tools for internal customers. Those people may have feedback/feature requests/etc which they want to send me. But I'm busy working on something else, and although input is good, we need to prioritize. So my manager takes on the task of processing and prioritizing input, leaving me free to develop.

My manager also has a role in shaping the broader roadmap, advocating for the team's priorities and leading us to a position to deliver on whatever the end result it, which may for instance involve prioritizing space for developers to learn new skills if they're going to be important to the roadmap.

So all that to say, I think there's a role that involves personal development of developers more than project management, and it makes things go a lot better when it's done right (and conversely, makes things worse if not).

As for quantifying it, I think technical debt (specifically, the variety that is actively hampering a team's development work rather than just "this could be better") can serve as an approximation. The team I'm now in had been poorly managed for a long time and was constantly buried in technical debt. We got a good manager, and those issues are drying up as we get the space allotted to solve the problems amid other feature work.