In Act 3 Scene 3, Why does Shakespeare “draw out” Desdemona’s request to her husband at such length? (Lines 53-86).

Desdemona's prolonged pleading to Othello on Cassio's behalf amplifies Othello's suspicions of an affair. Iago has already planted the seeds of discontent (Othello being a cuckold) and Desdemona's speech begins to drive Othello over the edge. This is exactly what Iago has planned.