An appropriate quotation can be a valuable addition to your presentation, for several reasons.
For one thing, a quotation can be a welcome change of pace from the standard information, facts, and statistics. You also build credibility for your case when you cite some authority as agreeing with you. (“After all, it was Albert Einstein who said, ‘Imagination is more important than knowledge.’ Who wants to argue with Einstein?”)
Quotations sometimes express an idea so succinctly, it’s unreasonable to try to improve on them. (“As Descartes once proclaimed, ‘I think, therefore I am.’”) And sometimes quotations are an excellent source of entertainment. (“As Oscar Wilde once said, “In this world there are only two tragedies. One is not getting what one wants, and the other is getting it.”)
On the other hand, quotations don't always work. It's disappointing to give your audience a tired old quotation they've heard many times before. And if you quote someone who the audience is unfamiliar with, the quotation loses its impact. And of course, your quotation should always be appropriate to whatever you're talking about in your speech!
Don't overlook the possibility of quoting yourself. This can be difficult to pull off, but if you can come up with a great quotation, you can not only get a lot of benefit from quoting yourself, but you just might find that other people start quoting you!