Obviously, it's more nuanced. But it's also really simple: designers should understand the final material in which their work will, well, materialize.
Say you're a carpenter. A true craftsman. You know all about the potential and the constraints of wood. You know how to design and make a great chair.
But now you want to make a thousand great chairs. You'll need to teach others your mastery. You better write some really good and detailed instructions.
And so you do. And because you know what you're talking about, your new crew trusts you.
♦ ♦ ♦
Snap. You're a designer. You don't know any code. How are you going to know what the best solution is? Let alone convince a whole team of engineers to follow your lead?