Confirmation or establishment makes a statement more firmly present to the audience. A familiar, often sensory image gives a conceptual assertion force that an abstract description might lack.
For example, likening futile effort to a vivid unsuccessful action does more than assert failure; it provides an image of failure and its outcome.
The effect can resemble a demonstration, but a simile is not necessarily a formal proof. Its chief mechanisms are visualization, clarification, and persuasion.