The reason I think it's good to have a product/project manager who can get down to this level if needed is that it increases the scope of possible solutions. W

The reason I think it's good to have a product/project manager who can get down to this level if needed is that it increases the scope of possible solutions. With a strict division between tech and product, typically the solution space is bounded by the product requirements. But if you have a product manager who is able to operate on the level of implementation details, you can adapt the requirements in ways which are better on both a user side and implementation side. It shouldn't be the norm that a manager is operating on the level of implementation details, but it is good if they can understand them, because maybe they have more levers than the development team has access to in terms of unblocking a tricky issue.