The form of a simile does not automatically reveal its purpose. It may prove possibility, clarify a state or degree, establish a judgment in the audience’s mind, beautify or disparage a subject, or present it as strikingly unusual.
كمن يرقم على الماء
Like someone drawing a pattern upon water.
This image makes fruitlessness vivid because the trace immediately disappears. When applied to an unsuccessful effort, its rhetorical function is not merely to name two objects; it makes the ineffectiveness of the activity easy to grasp.