On an iPhone, when is it typically essential to close a running app?

You must force-close an app on your smartphone if it is locked up. On an iPhone, you accomplish this by swiping the app up. Unless you've configured an iPhone app to refresh in the background, apps running in the background aren't really running or using resources. (Android apps fall short in this regard.) You might run out of storage space, but you probably won't get an "out of memory" error message. It's possible that your phone's battery is low or that you're out of storage space if it's slow. When you're done using an app for the day, you don't have to close it. They are not using resources if they are in the background.