Interesting. I model interior architecture as "here's $xxxK, make it nice" and they do a bunch of work to figure out what you mean by nice, and a bunch more work to codify your definition of nice into, like, SKUs of sconces and so on. Seems like NB helped you figure out your definition of nice, and your subcontractor had a good designer on staff to execute on that.
replyA sufficiently detailed render won't require a designer to figure out the materials. Any (reasonably competent) contractor can take a sufficiently detailed render with him to the store and find matching products. At least assuming the thing in the render actually exists.
replyHe can also send back a picture of the real product for approval. I think the primary difference here is the level of involvement. A quick consult and then the professional "makes it all work" versus hands on design with the client figuring out all the details for himself.