At some point along the way in your iPhone ownership, someone may have shown you that when you double click the home button, you can display your recent apps and, by swiping up an app screen, you can force quit specific apps.
Now, we don't want to take away anything from you. After all, inside information and tips & tricks is our thing. However, many people over/misuse this feature.
Your phone is smart, after all, so you don't need to regularly force quit apps manually. The iPhone software will manage its resources appropriately without your intervention. Doing so, for the most part, doesn't make your phone faster or operate better.
So why is the feature there? It's there for when an app is not functioning correctly, such as a non-responsive screen. It's not intended for digital housekeeping.