There used to be a whole bunch of "phone cleaner" or "RAM booster" apps like CleanMaster (ugh) that would claim to improve performance and battery life by killing background tasks. This would usually do the opposite, because Android is designed to keep RAM mostly full, so that the system can quickly switch between apps. Apps that are paused in the background do not use battery (unless they're designed to do something in the background, like a music app that's actively streaming). If you're constantly killing background apps, the system will keep trying to open those apps (or something else, based on usage patterns) in the background to prepare for usage -- it takes a bit of time and energy to reopen an app into RAM. So in the long run, the phone may end up a little more sluggish and use more battery.