Why did Edmund Spenser add a longer ninth line to the Spenserian stanza?

Edmund Spenser invented the Spenserian stanza for his epic poem The Faerie Queene to create a specific structural effect. While the first eight lines of the stanza consist of iambic pentameter that establishes a sense of formal unity, the ninth line—an alexandrine consisting of six iambic feet—serves to complete and resolve the thought of the stanza.1

By adding this longer final line, Spenser provided a sense of closure and rhythmic finality that distinguishes his innovation from earlier forms like the ottava rima or the eight-line stanzas found in Chaucer’s work.12