What is this thing that appears in Ctrl+Alt+P for? Seems useless?

Hi @dublove:

Adjust Layout was an early way to let a user to change the margins and columns document-wide, if, as per @rob day, frames were snapped onto the column and margin guides before implementing. It works as well today as it did back in Pagemaker, but these days is superceeded by the use of Primary Frames. We got a lot of questions here about people who want to change the paper size, margins, and columns for their longer documents. If they used primary frames, it's super easy but if not, they have to rely on Adjust Layout and hope for the best.

~Barb

Auto Adjust will only change the Document Setup margins. If you have manually changed a page’s margins as I have done here on pages 2 and 3, those pages’ margins will not change:

Before

After

Margins are just guides to give a visual to keep your work within. The document setup is where you set the margins. It looks like the Adjust Layout is to setup an automatic movement of items on the page if the page size changes.