Background App Refresh is confusing, and Apple doesn’t explain it well. To start with, most apps on iOS devices are “event driven”. That means that something happens that is of “interest” to an app, and the app runs to deal with the event. Examples: A mail message arrives, so the Mail app runs to slurp it up. Likewise if a text message arrives; the Messages app runs to add it to the message list. A weather update arrives, so the Weather app runs briefly to add the latest information to the weather app. The same idea for the Stocks app, Find My, Apple News, dedicated news media apps, Facebook, Snapchat, WhatsApp, etc. In addition, Notifications that you have enabled also cause apps to run to process the Notification, so you know, for example, that a new text message has arrived.
For WhatsApp specifically, it receives an “event” every time someone sends you a message, so for each messages WhatsApp “unsuspends” to decrypt and save the message to your iPhone.
All of these examples are apps that respond to an “event” that affects the app’s content. NONE of these "event-driven" apps are involved with Background App Refresh; they actually aren’t background processes at all, the are brief executions of the app in foreground behind the screen (if you are watching when an event arrives you will actually see the screen respond briefly). Background App Processing is used by apps that create their own events; for example, the Photos app periodically organizes the images in your photo library into automatically generated albums. This was not caused by any external event; the app just “decided” that it was a good time to organize albums. Or the Messages app takes time to “clean up” the message database. So it is only apps that decide, on their own to do some “work” that use Background App Refresh. Very few apps actually take advantage of this feature that was added several years ago. If you turn off Background App Refresh the apps that use it won’t do their “housekeeping” until you open the app, which may slow down your access to the app.
If you turn it off it will save some energy, but that energy will be used later when you open the app. So there is only minimal benefit to turning it off.
