They can quickly drain your battery
iOS 4.0 for iPhone finally brought multitasking to Apple’s device. The long awaiting background processes were finally allowed, such as playing Pandora radio will browsing through other apps. Before 4.0, only Apple’s native applications could perform background operations, such as the iPod app.
Along with the ability to background tasks, some apps can quickly drain your battery if left running. Especially apps that use your phone’s GPS or constantly stream data, such as Pandora. In order to save battery life, it’s best to close these apps completely.
To close an app running in the background, double-tap the Home button from any application or home screen. It’s best to do this from the home screen since you can’t fully close an app if you are in it.
Double tapping the home button will bring up the multitasking bar. From here you can swipe left or right and view all the applications still open.
Tap and hold your finger on one of the apps in the multitasking bar. This will cause the apps to wiggle and a red minus sign to appear on each one.
Touch the circular minus sign on each app to close it completely. It will disappear from the multitasking bar once closed.
Regularly closing unused apps will increase your battery life and phone performance.