Why is it being done?

Simply put, it’s to empower users. Rather than having a specific part of the site locked away in a theme or requiring a plugin, full-site editing will basically open it up to customize any part of your site the way you want to, or, on the flip side, you can let it be, or if you’re an agency, you could lock certain sites down or certain parts of a site down, depending upon how you want users to have access. So it’s a pretty powerful, big mission to actually deliver on full site editing.

Anne McCarthy

What is full site editing at a high level

Essentially, it’s a collection of features, and it’s important that you get the collection part. But it’s a collection of features that bring the familiar experience of blocks to all parts of your site, rather than just posts and pages.

Anne McCarthy

For the full transcript see Gutenberg Times Video: Full-Site Editing Overview with Anne McCarthy