The conventions serve two main purposes.

First, they provide an easily recognizable way for one to indicate which words and thoughts are one's own, and which thoughts and words are the accomplishments of another person.  Following the conventions is the only way to avoid misrepresenting another's accomplishments as one's own.  There are two important things to remember in this connection:  (1) Any part of your paper free of quotation marks and a citation is being presented as entirely your own words. (2) Any part of your paper that carries no citation is being presented as entirely your own ideas (or as common knowledge).

Second, these conventions provide a way to specify the actual source of thoughts or words that are not one's own. This provides proper credit to the other person, and indicates where readers can go to investigate further.