I normally keep the body element unstyled myself (outside of global tag styles) but all of my projects use Finsweet’s Client-First method. This gives me a page-wrapper class as my “root” element (just below the body) as well as a main-wrapper class as a child of the page-wrapper that holds all the primary page content outside of the nav and footer elements:
Image taken from the Client-First docs
Unless I’m needing to control everything on a specific page — including the header and footer — I’ll use the main-wrapper as the element that contains styles like display: flex.
It’s been a while since I’ve taken a peek at the Webflow 101 course so I’m not sure if they mention changing the display of the body, but in web development, there are tons of ways to accomplish the same goal. I would personally structure my page differently so I can style the main content with flex without changing the body style, but for someone just starting out it’s not necessarily the “wrong” choice.
Hopefully that answers your question, but don’t hesitate to reach out if you have any others 