There's two types of people:

> Those that relentlessly kill every single app, tab and task on their device.

> Those with 99 open tabs, that don't know how to kill an app. 

But who would win in a fight?

Generally, recent and running apps do not use up significant battery power or RAM. Your device continuously switches processes on and off as needed.

Don't think of it as running idle - rather, it's suspended. This means it is not using up system resources.

Killing an app before reopening it will place more strain on the processor than leaving it running. It's a bit like running in eco mode or hibernation - it's actually more energy efficient than switching off and on.

When you leave an app running, your device remembers where you left off. It stores this data in a memory bank called "Cache". When you open the app, data is quickly transmitted from the Cache to the CPU. 

This makes for a smoother, more efficient experience. For example, you're reading an article in your internet browser. You exit to send a message to a friend on WhatsApp, before checking Instagram. You come back to your article exactly where you left off. Imagine having to re-enter a search query, find the link, wait for the page to load, and scroll down to find where you left off.  

To bring up recent apps:

Old-school iPhones: Double tap the home button

New-school iPhones: Drag from the bottom and pause in the centre

Android setting 1: Tap the multi-window button

Android setting 2: Drag up slowly from the bottom

To clear the app, swipe it up.

When should you close an app?

> If it crashes

> If it's not responding or working properly

> You want the app to reopen on its home page with default settings

> It's running background processes that you don't need or want right now

> You will not be using the app again for a while

> You want to kill the app for privacy reasons

> Having apps open and running triggers your OCD

> You want to assert power and dominance over your apps

When should you keep an app open?

> You are switching back and forth with another app

> You want to pick up exactly where you left off

> It's running useful background processes, such as music playback or GPS navigation

> Literally, whenever you don't have an actual reason to close it

Apple and Android  both state that users should not clear their app history unless an app is causing an issue.

Well-designed apps will consume minimal resources when left open. Poorly-designed and buggy apps may drain your resources - this includes bloatware and, ironically, any third-party app that claims to speed up, optimise, or clean your phone.

Closing recent apps and killing background processes used to be essential for keeping your devices running smoothly. With modern phones; their abundance of RAM, and auto-optimising operating systems, there's no reason to clear all your apps regularly. You are free to enjoy switching between apps seamlessly.

Like biting your nails, this can be a very difficult habit to break.

TL;DR - Don't clear your apps for no reason, but it doesn't really matter that much either way.