كما في تشبيه من لا يحصل من سعيه على طائل بمن يرقم على الماء
As in comparing a person whose efforts yield nothing with one who writes upon water.
In the comparison, the image of writing that vanishes at once illustrates effort without a lasting product. The listener can readily picture this and therefore feels the futility of the activity more strongly.
The analogy should not be assigned indiscriminately to every rhetorical purpose. Writing on water is already familiar as a fruitless action: it is better suited to reinforcing an acknowledged situation than to proving the mere possibility of some strange event. Its immediate function is to clarify and consolidate the known failure.