The HCE architecture in Android is based around Android Service components (known as "HCE services"). One of the key advantages of a service is that it can run in the background without any user interface. This is a natural fit for many HCE applications like loyalty or transit cards, with which the user shouldn't need to launch the app to use it. Instead, tapping the device against the NFC reader starts the correct service (if not already running) and executes the transaction in the background. Of course, you are free to launch additional UI (such as user notifications) from your service if that makes sense.