In my work for a building product manufacturer, it is a daily struggle to figure out a designer's intent and to create a solution that actually works. The following, from an essay by Sheldon Wolfe, FCSI CCS, underscores this observation:

It is clear, however, that the countless products and the special knowledge they require make it impossible for a design firm to understand the construction part of architecture. Architecture schools do not teach much about building materials, structure, or systems, and they largely ignore construction methods, scheduling, and costs. Many have decried this lack of attention to the nuts-and-bolts part of architecture, but perhaps it now is simply impossible to teach all the things an architect would need to know to perform in the same way they did a hundred years ago, even with the intern development program.

Contractors, on the other hand, do know about construction, and that's what they're paid to know. Once merely workers hired to follow the direction of architects, contractors no longer rely on the architect to explain what has to be done. Instead, they now are expected to interpret the architect's documents and to determine for themselves what must be done to construct the building. They may know little about planning or design, but once construction begins, their practical experience, as opposed to the theoretical experience of the architect, becomes more valuable to the owner, and they are seen by owners as more realistic, more knowledgeable, even more important than the architect.