What is the purpose of scene two in Act 3?

Act III, Scene II

In this short scene Othello makes plans to inspect some parts of the fortifications built by his troops. The purpose of this scene is to explain why Othello is not initially present when Desdemona meets with Cassio. Iago had planned to concoct a story to ensure Othello was absent for Cassio's visit, but luck has made Iago's job easier indeed.