I’ve been blessed to have worked as an engineer, an engineering manager, a product manager and a “product manager manager” alongside leaders and peers who “did product right” and am aware enough of companies and people who “did it wrong” to compare and contrast.
At the end of the day, someone needs to have a vision of how your product is going to be successful and can walk the team along that journey. Back in the day, and in small companies today, that person is often the engineering manager or even the technical founder. In large companies, this is the primary response of the PM.
The reason is that engineering managers who can also “do the product” thing are rare, both because there’s certain personality traits and skills that are required but not common and it takes a lot of bandwidth.
For example, in my first job as a PM, I traveled to 20+ countries every year meeting with hundreds and not thousands of clients and prospects to really feel out my product’s place and opportunity. That investment of time and breadth of “perception” put me in a really good place to lead my product.
It’s hopefully obvious that I wouldn’t be able to do that and also be responsible for careers of 50+ developers, technical direction etc, so yeah I had to be a PM, not an engineering manager, to operate like that.
In terms of impact, the most junior PMs take load off engineers. Eg: they go to the customer and write down what they ask for. The engineer could do that themselves but it’s deemed a better investment of their time to keep coding, so the junior PM is thrown at it to free them up. A middle level PM talks to a bunch of customers and defines a solution for their biggest problems (in contrast to the junior who depends on the customer to tell them what is needed in what order.) This PM synthesizes customer information into something different and hopefully easier to deliver and more scalable. This is where the skill set diverges from engineers. At the most senior level, the PM anticipates the kinds of problems their customers will have down the road and acts as a “visionary” to get the solutions built proactively.
PMs obviously can be asked to do other jobs, like I’ve often seen them tasked with running sprints etc. it’s fine if the PM or someone from engineering does it but in neither case is it a required part of the job and is better done by a project manager who is more inanely focused on running processes.
Another way I like to describe what a PM does - they are the teams interface into the world, on the hook for turning external ambiguity into internal clarity. They are on the hook for driving priorities with the team and that doesn’t just mean customer priorities. For example, sometime a purely technical thing can be the most important thing to work on and at other times the same thing could be a waste to address. The PM should be able to help the team understand which situation they are facing.
All this btw explains the massive pay disparity in PMs across seniority and across companies. Some companies view PM as that “leverage to engineer” model I described above which by definition pays less than the engineer. On the other extreme companies see senior PMs as the strategic leaders of multi-billion dollar product lines.