Android Marshmallow has one more change that promises to improve app management: a new, one-stop shop for setting default apps for various tasks. Using the new Default Apps settings pane (Settings > Apps, tap the gear icon, then tap Default Apps), you can change your default voice assistant app, browser, phone app, and SMS app. It’d be great to see more options in the future, and it’s too bad this useful feature is buried in the Settings app.
The gear icon in the Apps settings houses a few other advanced features, such as whether an app can "draw" over other apps (that is, whether an app can display information atop another app) or whether an app can modify system settings. As Phandroid points out, though, it's still unclear why these features are here, so we'll have to wait and see.