From its earliest period, long before Freud knew what he was doing or where he was going, he used the metaphor of wearing away to express the therapeutic effects of the talking cure. Freud's concept of the unconscious is poised on an opposition between the durable [in the unconscious] and the mutable [in consciousness]. What is unconscious is timeless, of stone, forever, while what is conscious is transient, ephemeral, written in water. [...] [T]he distinction between what is conscious and what is unconscious remained, and remains, a kind of lodestar of psychoanalytic thought [...]and the wearing-away metaphor retained its authority as other figures of pre-psychoanalysis lost theirs (p. 4).
What I think is significant here is that it succinctly gets at the difference between the conscious mind, which is privileged by most people, and the unconscious mind, which is the focus of psychoanalysis.
It also provides a short rationale for why psychoanalysis privileges the unconscious content over conscious thoughts.