The idea is to have a monitor app with which you can connect to an MQTT broker, subscribe to a number of topics and publish a text payload to a topic or two for debugging purposes.
The result is a pretty versatile utility for your MQTT toolchest. It serves as a viewer with which you can observe payloads being published to topics you're interested in, and you can publish payloads to topics at the tap of a button. The app is designed to work on both iPad and iPhone.
With Family Sharing set up, up to six family members can use this app.