What is the principal role of a metrical foot such as fāʿilun in prosody?

Division of Surahs into Verses, Qur’anic Rhythm, and Verse Endings

• To represent the syllabic and rhythmic pattern of speech
• Each metrical foot identifies the rhyme’s rawī
• A metrical foot formally measures the semantic fit of an expression
• A metrical foot only measures the final word of a hemistich