That, tossing anything an applicant submits with the application "into trash bin," would be a blatant abuse in breach of fair procedure requirements. Might happen sometimes (stuff happens), but almost certainly that would be the exception, a very unusual exception.
Does not mean the additional material is considered much.
The extent to which unrequested documents or letters or other additional materials submitted with an application are "read," or in any significant way considered, is an open question, but it is almost certain that it varies considerably from applicant to applicant, depending on what it is and the context.
Sure, additional material may indeed be disregarded . . . BUT NOT discarded.
What applicants elect to include with their application, in addition to what is specifically required, varies widely. Among the more common approaches, some applicants include what they call a "Letter of Explanation" (LoE).
The conventional wisdom is to go sparingly, if at all. Generally, similar to interacting with border officials during a Port-of-Entry examination, the prudent approach is to answer the questions asked, as asked, and not volunteer additional information. Generally no "explanation" is necessary, or even helpful, UNLESS the official handling the matter actually has a question, which is indicated by an actual question.
Attempts to preempt issues by submitting information or documents not requested are typically futile (which I gather is the gist of what @APPNOV2014NY was saying, noting again, however, that disregarding such material is not the same as discarding it) and if the extra material or information has any influence, the risk is that it raises questions or issues more than resolves them.
Like explaining a move out of province: generally the total stranger bureaucrats processing a citizenship application are not looking at or concerned with why an applicant moves about particular locations in Canada, at least not much beyond contextual information sufficient to verify the completeness and accuracy of the applicant's address and work history. So, whether in a "cover letter" or a LoE, volunteering information about reasons for moving from one province to another is far more likely to draw attention, potentially raise questions, than it is to settle any concern about misrepresentations the PR made in the course of becoming a PR.
There are situations in which adding a LoE makes sense. So it would be imprudent to say never do that. But again, the conventional wisdom is to go sparingly . . . meaning both not adding information or material unless there is a clear need to clarify or explain something, and to be very brief if any additional information or material is submitted.
The risk is bringing up something that would not have been an issue otherwise. While, in contrast, if it is about an issue that is likely to come up anyway, the additional material or explanation will rarely preclude non-routine processing to address that issue.
