“Manufacturing” is code for “the folks who will produce the thing at scale.”
It’s easy to think that the first version of an invention works consistently, you can just turn around and make a bunch of them. However, the way something is made has a huge influence on its cost. A piece of metal shaped by hand by a skilled craftsperson is much more expensive than that same piece of metal cast in a mold. Sometimes it’s straightforward to turn the design of the former into a design for the latter. However, sometimes you need to redesign the thing almost from scratch.
The difference between these two situations is often not obvious to someone without manufacturing experience. There are many non-obvious design choices that make scaling easier or harder. For example, right-angle corners at the bottom of a cut-out are completely free (and the default!) in a CAD model, but are extremely hard to machine correctly. In addition to the non-obviousness of what can stand in the way of scaling, it’s often Tacit Knowledge rather than some legible list of “do this, don’t do this.” Someone with a lot of experience will take one look and say “mm nope that’s a bad idea.”
The importance of having manufacturing in the room applies in software as well as hardware. Different implementations of the same algorithm can parallelize fine or completely break, a functional piece of code could leave giant security holes, etc.