Primarily, it is to avoid allegations of plagiarism and academic misconduct.
Secondarily, citing sources allows the reader to identify the sources that you relied on for your research.
It's easy: whenever you paraphrase the ideas of others, whenever you use a direct quotation, or whenever you cite a fact that isn't common knowledge, you need to cite your original sources, whether they are books, articles, or web site.
The keys to successfully citing your sources in APA Style are:
APA recommends that legal materials (such as cases, statutes, or collective agreements) be cited according to The Bluebook: a Uniform System of Citation. The Canadian equivalent to this U.S. legal citation guide is The Canadian Guide to Uniform Legal Citation. However, as it is silent on how to cite a collective agreement, I propose the following example:
Brock University and the Brock University Faculty Association (2014). Collective agreement between Brock University and the Brock University Faculty Association, July 1, 2014 - June 30, 2017. St. Catharines, ON. Retrieved from:http://www.brocku.ca/webfm_send/33840
The following links provide guidance on citing legal materials such as cases and legislation according to the Canadian Guide to Uniform Legal Citation (aka the McGill Guide).