The idea is that apps in the background are locking up unnecessary RAM and consuming unnecessary CPU cycles, thus hurting performance and wasting battery life.
That’s not how iOS works. The iOS system is designed so that none of the above justifications for force quitting are true. Apps in the background are effectively “frozen”, severely limiting what they can do in the background and freeing up the RAM they were using. iOS is really, really good at this. It is so good at this that unfreezing a frozen app takes up way less CPU (and energy) than relaunching an app that had been force quit. Not only does force quitting your apps not help, it actually hurts. Your battery life will be worse and it will take much longer to switch apps if you force quit apps in the background.
Here’s a short and sweet answer from Craig Federighi, in response to an email from a customer asking if he force quits apps and whether doing so preserves battery life: “No and no.”
In fact, apps frozen in the background on iOS unfreeze so quickly that I think it actually helps perpetuate the myth that you should force quit them: if you’re worried that background apps are draining your battery and you see how quickly they load from the background, it’s a reasonable assumption to believe that they never stopped running. But they do. They really do get frozen, the RAM they were using really does get reclaimed by the system, and they really do unfreeze and come back to life that quickly.
An awful lot of very hard work went into making iOS work like this. It’s a huge technical advantage that iOS holds over Android. And every iPhone user in the world who habitually force quits background apps manually is wasting all of the effort that went into this while simultaneously wasting their own device’s battery life and making everything slower for themselves.