When should Background App Refresh be turned on or off in Settings?
I don’t know how much Background App Refresh drains the battery or other resources, but would like to keep it on for some apps where it might be important, and off for the rest of them if there’s no benefit.
If you go to Settings, General, Background App Refresh, you can do just that. You can specify which apps use it and which don't. You can also set it to only work when in Wi-Fi.
Sure it will use a little battery (everything you do on your iPhone uses power and this is no different). So it is whether it is worth it or not to you. I'd expect that most apps barely use any power just to check for updates to data. But it is hard to know how each app uses this.
I simply have it on for everything. I expect my iPhone to be useful and up-to-date. That's what it is for. But I never even get near emptying the battery on most days.