تَوَقَّفَ الدَّلِيلُ احْتِيَاطًا، لَا جُبْنًا.
The guide stopped out of caution, not cowardice.
The first clause might be taken as suggesting timidity, especially where the guide's leadership is in dispute. Lā jubnan blocks that unintended implication and functions as iḥtirās: protection of the intended meaning against a foreseeable misreading.
If cowardice is not even a plausible inference in context, the phrase may seem needless. The appropriateness of protective qualification is measured against the hearer's likely understanding.