In either writing or speaking if you do not acknowledge your sources of information, your audience is led to believe that everything presented is your own work. Citing helps your audience differentiate between your own work and that of your sources.
Using the work of experts can increase your credibility but for that to work the audience has to know that you are using the expert's work and be able to assess the credibility of the expert.
For instance if I tell you that in 2016 Social Security is going to start spending more than it takes in, you must assume that I am making that statement from my own knowledge, and what do I know about Social Security spending? However if I tell you that in a July 2010 report the Congressional Budget Office (a non-partisan Congressional agency charged with evaluating Congressional proposals for their financial impact.) projects that in 2016 Social Security is going to start spending more than it takes in, you can evaluate whether you trust the Congressional Budget Office as a source of information. You can go and find that report and read the statement for yourself and decide whether I have interpreted it correctly.
Fig. 1: Rcragun. "Social Security Benefits - 2009-2083.png." Chart. Wikimedia Commons, 7 Oct. 2009. Web. 16 Aug. 2010.
Citation also gives your audience the means to find your sources.
Have you ever listened to a speech and thought that a particular web site, article or book sounded really interesting and wanted to find it for yourself? Conversely have you ever heard about a particular web site, wanted to read it, and found that the person who told you about it did not give you enough information to actually find it?