What is the function of “the day of Fiṭr is a day of rewards” here?

Narrator Criticism, Integrity and Precision: Standards of Scholarly Appraisal

• Its appealing moral meaning plus Ibn al-Mubārak’s mention authenticates the chain
• Naming Sulaymān b. al-Ḥajjāj establishes every condition of the attribution
• Attribution to a Companion removes any need to examine later links
• It is a cited attribution under scrutiny, whose transmitter Sulaymān b. al-Ḥajjāj is identified