Closing backgrounding apps costs batteries 

Smartphones are smart little computers, however, unlike your laptop, having a bunch of apps open at once—especially ones that connect to the internet—strains your battery, not so on smartphones.

In the case of iOS, apps do not stay open the same way they do on a computer. When you leave an app, it’s frozen so it doesn’t do anything, and doesn’t require any resources. Closing the apps costs CPU power and battery to close the app, you are also taking the app out of the devices RAM so 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 which costs even more power.

Source Lifehacker.