لِئَلَّا تَصِيرَ الْاسْتِعَارَةُ إِلْغَازًا
So that the metaphor does not turn into a riddle.
If the resemblance is neither immediately apparent nor supported by shared custom or the audience's specialized knowledge, the figure can become difficult to interpret. The listener must first invent a link between apparently unrelated things rather than receiving the intended meaning.
Subtlety may be an advantage, but obscurity by itself is not eloquence. Sophisticated meaning should be distinguished from an unintelligible puzzle: a successful image illuminates its purpose instead of hiding it.