Imagine you're a project architect and you're nearing completion of CD's. For half a year, you've been neck-deep in the project meetings, made myriad decisions large and small, and are intimately knowledgeable of the design intention and you know all of the details like the back of your hand. All that's left for you to do before the job goes out is write the specifications.
There are a number of important reasons why, even with all this knowledge of the project, you're not the right person to write the specifications.
Specification-Writing Experience
As a project architect, you may write a couple of project specifications over the course of a year. A busy in-house specifier or a specifications consultant are likely writing a full project specification every week. That volume matters when it comes to understanding construction materials and systems. An experienced specifier will have done more research and will know why requirements are included in commercial master specifications, and where those requirements are applicable.
As an example, we just received a client written office master specification that included an ASTM reference that had been withdrawn and replaced, years ago. The obsolete reference standard included products that would not comply with current building codes. Full-time specifiers do a better job keeping up with industry reference standards, so the specifications are not riddled with incorrect references.
Another Set of Eyes
An independent or dedicated in-house specifier probably will not have seen the project until late in the development of design development or construction documents. The specifier is experienced at digesting the contents of the drawing set in order to begin writing specs, and at the same time performs a valuable quality assurance service. While reviewing the drawings, the specifier will flag instances where the drawings are inconsistent, incorrect, or incomplete and communicate recommendations to the architect, offering the architect the opportunity to improve the documents.
Specifiers' QA review frequently turns up constructability issues, gaps in building envelope continuity, missing (or superfluous) elements in assembly details, code questions, and drawing coordination issues.
Owner's Advocate
While reviewing the drawings and writing the specifications, the specifier will likely spot details that, while technically correct and clearly representing deliberate design choices by the architect, will result in higher construction cost relative to the general quality level of the rest of the building. The specifier may simply ask, "does the client understand that this detail costs more than a standard detail that will perform just as well?" An owner that isn't asked to address design options posed by the architect during design will almost certainly have them brought up by a contractor during construction.
Summary
Architects generally wish to avoid awkward Owner-Architect-Contractor meeting conversation in which they are forced to admit errors or explain over-designed details. They can do little more than use experienced specifiers to help themselves prepare documents that have been troubleshot in advance and are correct and coordinated.