I don't know the intricacies of building construction.  For whatever reason they might have a better price on a slightly different size.  Going slightly wider/narrower/longer/shorter might be a better deal. It could be I have something specific I want (say a minimum size door) and changing the specs can make it more affordable or stronger. That is why I talk to someone in the business instead of just doing it myself.

It's all in how you approach things.  If you know everything then you can specify exactly what you want and it will be perfect.  However my experience has been I am better off giving a salesman an idea of what I want and let him offer suggestions on a better way of achieving it.

Having said all that, it is important to stay focused on things you consider essential and not let the salesman talk you out of them.