Very often we do several mockup pages with same elements. For example, if we create website mockup - site header, footer, menu, sidebars are same for all pages. If we create application design - menu and other interface elements must stay here on all pages.
At now we must create a copy of master page and place elements over it in new layers. But if we need to change something in master page (for example, rename menu item) - we must do this repeatedly on each page.
Will be good to have "Specify master page" function for each page in document - draw.io place it as readonly background (read-only layer) for current page, and update this background if master page changed.