Richard,
You're right that we can do so many things using Storyline graphic tools, very similar to PPT. Yet, as Alexander points out, PPT has a few additional tools and effects which offer more functionality and allow for some creativity.
I sure agree with that and offer a couple of thoughts:
1. prototyping your design and graphic elements in PPT using the same screen size and proportion will save you time as you are building your Storyline file. You can try out multiple layouts and color palettes very easily in PPT, then transfer the elements to your story file.
2. those layouts and graphic elements can then be stored within a PPT file or series of files for future reference and re-used for your future projects.
Overall, I have found I can visualize and preview my screen layouts and colors more rapidly in PPT than working exclusively in Storyline.