Byron uses the final couplet in each stanza to
a. create a punch line.
Byron uses a specific poetic structure identified as Ottava Rima. It has a rhyme scheme of ABABABCC where each letter stands for the lines' end rhyme. Its rhythmic pattern is in iambic pentameter.
The couplet is represented by CC. These lines give a humorous conclusion to the six lines that come before it. In short, these lines are the punch lines.