The new query planning model in Vitess aims to simplify the optimization process by breaking it down into smaller, manageable steps, each resulting in a runnable plan. This approach offers several benefits, including improved visualization, increased testability, and enhanced flexibility with expressions. By making every step of the optimization pipeline a runnable plan, developers can better understand and reason about the impact of each optimization step, leading to more efficient and optimized queries. The model also allows for arbitrary expressions to be used in ordering, grouping, and aggregations, providing greater flexibility when crafting complex queries.