The old menu button has been long forgotten. This was a staple on older Android devices. These days the menu is integrated as a hovering 3 dot dot, or a 3 dot button in the top of your app. The LG G4 still makes use of the menu nav button. If you need to access the menu from the homescreen or from within an app by long pressing the recents button. This can really come in handy when you are using the G4's stock apps. Bringing up the menu from the stock launcher gives you quick access to home screen and lockscreen settings. This feature is slowly being fazed out, but it can be really useful at times so it's nice to see that they G4 has not ditched it completely.