By closing the app, you take the app out of the phone's RAM . While you think this may be what you want to do, it's not. When you open that same app again the next time you need it, your device has to load it back into memory all over again.
Plus, iOS closes apps automatically as it needs more memory, so you're doing something your device is already doing for you. You are meant to be the user of your device, not the janitor.