As you mention, it streams apps from the phone (iphone or android) or android stick. If you use only the licensed App Radio app, you are limited to what apps you can use. If you do not connect a phone or android stick the unit basically useless, as you will only be able to use the radio, and hands free system. In addition, you can connect up a wireless/wired camera.
To get the full potential from the unit, you are best to connect to an android stick. Using the screen mirror function it gives you the android interface on the head units screen. Bare in mind without rooting and installing the app radio unchained app, you will not be able to touch the screen in order to navigate/use any of the apps. You can connect a mouse, or use the remote control that is provided however this would be pointless in my mind.
Really simple steps to root the device and install the other app from the Play store. If you check out Vin's mod thread, he has some videos of the kit in action, YouTube is also full of demos with the android software on the unit. I have found some tweaks that are not in the guides online that speed up the connection time - under 20 seconds. I use my phone as a wifi hot spot which then gives the head unit internet access - so can go online, use Google maps, sky go, TV catch up, spotify etc. And with it being android UI - you can customize it in pretty much anyway you want.