[T]hat part of the soul in foolish people where the desires reside—the uncontrolled and nonretentive part—he likened to a leaky jar, because it can never be filled.

Socrates utilizes this leaky jar metaphor (493b) in order to display the significance of temperance, a driving theme of the dialogue. Essentially, Socrates intends to prove that the control of (rather than a giving-in to) one's desires brings a person closer to virtue. This metaphorical imagery comes on the heels of Callicles's proposal that proper living results from one's possessing the bravery and intelligence to satisfy desires. It illustrates that a being who continually stokes the fires of his or her appetites will never be able to quell their ever-growing want and need. Just as a larger hole means more can get through and thus requires more to be filled, so too does a stronger desire require more and more for its satisfaction.

The claim here marks the beginning of a crucial argument within Gorgias. Socrates ultimately means to tout temperance and justice as the main aspects of a good life, a notion that itself is (or, at least, should be) the highest aspiration of any human. By so vividly depicting the nature of a soul (jar) without control (leaky), a vital point becomes quite clear—so much so that even Callicles has no choice but to agree.