“Zayd is generous” states the quality directly. “Zayd is generous like a river” presents the same quality through a concrete comparison. The central subject remains generosity, but the second formulation supplies a vivid image to the listener.
Clarity of signification does not require every sentence to be maximally plain. An apt comparison or allusion can express a thought more vividly while remaining intelligible. The chosen form should lead its audience to the intended meaning.