In former times, Windows programs such as Word or Excel or Quattro Pro all had two windows: an outer application window and an inner document window. (We Mac users thought it was cumbersome and silly). Closing the last document window still left the application running, and the screen would be taken up by the blank application window until you either closed the app (by closing its window or otherwise) or opened a new document. You could also Alt-tab to some other application and leave Excel running.
Nowadays, many Windows programs (including MS Office apps) no longer have the application window.
That apparently means that when you close the last document window (like, say, a letter written in Microsoft Word), Word itself exits. (On the Mac, most applications remain running even when the last document window is closed; it leaves nothing behind except its menu set until you open another document).
I assume it is inefficient to exit out of an application program if you’re going to want it launched again immediately to open a different document. Is modern Windows actually doing that when you close the last document window? I can’t tell for sure – if I double-click a .docx document on the desktop or in my documents folder, I get the Word splash screen as if the application were launching even when other documents remain open already. But on the task bar at the bottom there’s no longer an underbar after the last document window closes.
Is the spread of SSD drives making all of this somewhat irrelevant and Microsoft wrote its OS in anticipation of not having to care one way or the other or something?