I’m a late-comer to working with Tabs in Dorico, which are fantastic for viewing different parts and scores. Particularly in large documents, it seems quicker to switch between tabs of different layouts than to switch the Layout in the same tab/window.
You can also have separate windows of the same document, too. There may be some significance I’m missing to the difference.
However, this might be a bug: If you’re in Engrave mode, you can switch to another tab that displays Galley View, and still be in Engrave mode.