1 voteNathan Pyritz commented
Adjust Layout is new to CC 2019. I haven't personally found any reason to use it yet. It seems to be for cases when you need to change the size of a document. From my point of view, this would require changing/adjusting so many details that I would not feel comfortable automating it. I would want to adjust everything manually, to make sure every detail was addressed, adjusted, and locked down how I wanted.
Layout Adjustment is a feature that was REMOVED in CC19 that will adjust the contents of your page (according to a few straightforward parameters) if you change the margins (or, I suppose, the document size). When working with large multi-page documents with multiple master pages (regular text page, short/long text page, chapter openers, part openers, etc), Layout adjustment made it quick and simple to start a new chapter by applying a chapter opener master page. The text boxes on the page would simply and automatically resize to fix the new margins. No need to manually close or open up text boxes every time. It also made things super simple when you're still in the design stage, trying to work out the ideal margins. If you decide to need to make the outside margin a few points wider, you just had to adjust the master page, and text would all follow suit. On a 400 page book, this is indispensable.
For book design, I'd consider Layout Adjustment one of the top essential tools in InDesign. Please bring it back.