One of the recommendations of IEEE 1471 (the standard for describing the architecture of a software-intensive system) is to establish a conceptual framework and vocabulary for talking about architectural issues of systems. To provide a definition of what controlled vocabularies are and why they should be used i refer to Wikipedia and the following quote:
In organizations, controlled vocabularies may be introduced to improve technical communication. The use of controlled vocabulary ensures that everyone is using the same word to mean the same thing. This consistency of terms is one of the most important concepts in technical writing and knowledge management, where effort is expended to use the same word throughout a document or organization instead of slightly different ones to refer to the same thing.
Both enterprise architecture and evidence based activities benefit greatly from a controlled vocabulary. By creating and agreeing on a controlled set of terms, and the meaning behind the terms, helps readers understand commonality and relationships across document sets that support projects, processes, policies and strategies etc. and therefore a greater understanding of what the document is trying to say.
Where evidence is used to support enterprise artefacts and documentation the use of controlled vocabulary helps ensure that the evidence is understood and bears meaning to the hypotheses and recommendations it is supporting.