Nouns are Objects
Most likely Nouns in this context are entities which exist in the task domain. I see direct parallels between Noun and Object in OOP terminology.
An Object incapsulates attributes and methods. Noun contains Attributes and Actions. For visualization Noun use Views. So the Nouns are abstraction tool which is convient for analysis of some domain.
When Nouns are useful?
In user-centered design primary task is to define user needs. One way of doing it is direct observation and task analysis. When you have no direct access to users or just to complete a picture you could also analyse the results of users' activity, users' tools and other artifacts. So this looks like a kind of reverse engineering.
Primary Noun Table is a system of "nouns" as defined by interaction objects and tasks to communicate the implications of a design, it's components and it's behaviors. "press play to begin the movie" - this is, believe it or not, a UX/UI component behavior instruction.