I guess these just enable web developers to try out new features to embed in their webpages. Thus they can prepare their websites for future technologies and in later iOS / macOS revisions these will be enabled by default. Also they might be additionally included in some HTML / Web Standard eventually.
As a user, you won't be able to see much of a difference unless the websites you visit specifically supports one of these features. This possibly holds true for CSS-related functions, you might however see some performance improvements when enabling WebGPU or Async Frame Scrolling.