Roughly speaking it is the difference between having a contractor go out and do some work and having that same contractor first come up with a plan to do some work, run that by you, and then go out to do that work.

Part of it is as a another comment in this chain mentions the chance to review the prompt. Part of it is that it forces the AI system to plan things in a certain order, in much the same way that forcing the contractor to write the plan out first forces the contractor to proceed in a certain predefined order that may (or may not!) be better at getting to a final answer.

The ability to correct the plan/prompt.