The problem with using an app is that most of the less expensive zwave switches do not 100% support instant status updates. SmartThings has some workarounds in place to “help” with this. @Duncan and @chrisb do a great job at outlining the limitations in this post:
The other nice thing about device to device association is that it will work without a controller. So if my internet/hub is offline, all the physical switches will still work. Until presence detection is vastly improved and the automation is to the level you don’t even have to think about lighting, dependable physical switches for me are a must. It is hard to accept an inconsistent experience for something as basic as a wall switch. The hard-wired Aux switches are also and option, but I haven’t found any dimmers that work well with LEDs.