In short, citation brings into play a complicated dynamic of authority, competition, novelty. The point here is to be self-conscious about how you deploy other critics' writing. What are you doing by quoting someone else?
Establishing what the critical consensus is.
Finding support for something that your audience might not agree with.
Defining the position with which you disagree, either partially or wholly.
Establishing contextual information.
If you know why you are citing then you can figure out how to cite.