The beachball means that the aplication/thread under the mouse pointer is not responding to UI events.
If you see it when the pointer is over a particular application, then that app isn’t responding. If you see it over Finder windows or the desktop, but not other apps, then the Finder is not responding. If you see it everywhere, then it could still be the Finder, but might be something at a lower layer of the system.
As for why this happens, it could be many reasons. As you’ve mentioned, disk and network problems activity are the primary cause - the UI thread is doing something that normally finishes quickly (e.g. opening a file, or doing a DNS lookup), that ends up taking a long time for some reason (e.g. disk powering-on or failing, network congestion/failure).
But it could also be an application bug, where the app is doing something that always take a long time and should never be done from a UI-event thread.