It is quite common to see iOS users quit all their apps in the multitasking view in a bid to save battery life or to improve speed. This is especially true if the users came from using Android. iOS however has a different ways to handle background apps, and it is completely unnecessary to close every app frequently.
iOS has strict restrictions on what an app can do when it is in the background. Only few categories of apps are allowed to run in the background, those are apps that needs location updates (e.g. running apps), music playback, and apps that refresh its data from servers using Background App Refresh feature. For all other apps, they are frozen in RAM when in the background and are not using up any battery.
To reduce location updates usage in the background, go to Settings > Privacy > Location Services and only enable access to those apps that you want to share your location.
Similarly for background app refresh, go to Settings > General > Background App Refresh and only enable those apps that you absolutely need the latest data available all the time.
What’s killing your iPhone’s battery in the background are apps that are abusing what Apple allowed apps to do. For example playing silent music (so that it can be run in the background) but doing other craps in the background. Or it might be caused by some bugs in the app.
Thus there is no need to cultivate the habit of quitting all apps frequently. If your battery is draining abnormally, go to Settings > Battery and check the battery usage to see if there is any rarely run apps that uses large percentage of battery.