You can open additional windows for the same document or for other InDesign documents. With additional windows, you can compare different spreads simultaneously, especially spreads that aren’t adjacent. You can also display different magnifications of the same page, so that you can work closely on details while watching how the changes affect the overall layout. In addition, you can display a master page in one window, and pages based on it in other windows, to see how editing the master page affects various parts of the document.
When you reopen the document, only the last-used window appears.