iOS was made to be an efficient multitasking tool, so when apps are not currently running on your iPhone, they are put into a type of standby mode that isn’t using CPU resources. Issues, however, arise when you eventually reopen a closed app, as additional power and memory need to be used to properly start the application, which can cause your battery to drain.

As long as you turn off Background App Refresh — the feature that allows apps to check for updates while they’re not in use — in the iPhone’s Settings app, you don’t have to worry about closing background apps because they aren’t technically running in the first place. Doing so can save a lot of your battery life and will make multitasking operations on your iPhone hassle-free.

The only time you should close an app is if it has become unresponsive, either frozen, not working correctly, or otherwise disabled. At this point, it's a good idea to close the app in question and attempt to restart it, which you can do on the iPhone X and later models by swiping from the bottom of the screen, finding the unresponsive app, and swiping up to force close it.