They allow the reader to quickly and easily locate a source in the reference list at the end of a work (in most parenthetical referencing formats this is labelled "References" or "Works Cited" and includes only sources directly cited in the written work), with a minimal disruption of the flow of the writing in which they appear.
This does not just refer to direct quotations but also to paraphrases, data, and even broad "schools of thought", or ways of thinking about a topic.
Plagiarism is not always dishonest – you may not be intentionally passing off someone else's work as your own – but if you don't cite any source that may have influenced your thinking you're open to plagiarism accusations.
Page referencing conventions can vary considerably from format to format, depending on the discipline in which a referencing style is generally used.
APA, for example, is designed for use in Social Sciences contexts and doesn't have a straightforward built-in mechanism for specifying page numbers, since it is expected that the vast majority of the time it will be used to reference paraphrases and summaries rather than direct quotations.
Specifically, these have to do with your development as a scholar and your participation in the collective creation of knowledge: In short, you may not have much of a choice.
How Do You Reference A Thesis
If you're lucky your university will let you choose between their preferred styles of parenthetical referencing and footnote referencing (for Arts or Humanities subjects) or their preferred parenthetical and numerical styles (for Sciences or Social Sciences).
When using APA to provide direct page number references it is recommended that the author and date be included when introducing the citation and the page reference be provided in a separate parenthetical note at the end of the citation; Smith (2012) argues that "There are far too many referencing styles for the young scholar to keep track of" (p. Harvard deserves a special mention among author-date systems: while it's very likely that your university uses Harvard for referencing in at least some disciplines, it's also equally likely that your university's idea of what Harvard referencing looks like is slightly different from any of its neighbouring universities.
Whereas referencing styles like Chicago and MLA form part of detailed style guides that provide explicit rules on many aspects of scholarly writing (not just referencing), Harvard simply defines the types of information that should be included in a reference and some broad principles about formatting.
But by far the most efficient way to store, retrieve, and cite the sources you find – especially if you're working with lots of recent, online sources – is to use reference management software.
Universities often have subscriptions to commercial packages like Ref Works and End Note, but you can still save a lot of time with a freeware package like Zotero (though it has far less sophisticated bibliography-generating tools, and with far fewer citation formats, than the commercial tools).