The on-canvas menu lets you review an outline before any slides exist. Above a slide, you open the Copilot menu, choose "Create a presentation about..." and describe the deck. Copilot first proposes a list of topics. You can add, remove, reorder or regenerate any topic with the pencil button, and only then select Generate slides.
That is the main difference with the other prompt route in Copilot in PowerPoint, the Copilot button on the Home tab, which drafts the whole deck in one pass: faster, but with less control. The same menu is where you refine afterwards, for example to rewrite a slide or change the speaker notes. On-canvas creation was still rolling out to Microsoft 365 Insiders, so it may not appear in every tenant yet.