An especially important purpose is implying the grounds of the predication (الإيماء إلى وجه بناء الخبر). The relative clause states a characteristic that makes the following predicate intelligible or fitting; the audience infers the connection between them.
Example:
«الذي يحفظ الأمانة محبوب» — The one who safeguards trusts is loved.
Trustworthiness readily appears relevant to being loved. Instead of explicitly saying “because he is trustworthy,” the speaker uses the relative clause to suggest that relation. The rhetorical effect lies in the implied reason, not merely in the existence of a relative word.