Hard to say what its function would be when attached to the main controller. Although I think it's pretty safe to assume they wanted the "left joy-con" and "right joy-con" to have the same number of buttons, given the local multiplayer feature. I'd also say it's safe to assume that each side needs a button which can more-or-less function as a "start" button. Traditionally this has been +, so I expect Nintendo to stick with that trend and use + and -. That leaves really only one logical option, and that would be that it mirrors the home button functionality (when playing multiplayer with detached controllers, at least.)