I guess Android would like to fill available RAM with apps, for quick loading and other reasons. Best that these apps are the ones you frequently use. Hence a reason to close apps that are installed but are used very infrequently (if ever). Make sense?
That idea makes sense, but it's not how the system works. Force closing apps will often lead to them restarting automatically, which takes power. Just use your phone normally. If there's an app that's always opening and that you never use, Disable it.