There is no fuel gauge - you have 2hrs endurance, with FS default fuel you have 1. Challenge: Find out if that’s true! 
I guess that’s really the only purpose of the timer, which seems to be only kind of working - once you stepped through to the timer (confusingly with the “start/stop” buttons) you can click the ‘menu’ button to toggle between the “H1” and “H2” timers and holding that button for a while kind of resets the H1 timer. Asobo does not provide documentation and even the Top Rudder website doesn’t state the make/model of this wonderful compact electronic thingamajig so we’re on our own to figure it out.
The engine has dual electronic ignition and the buttons simply “solo” the corresponding ignition circuit/coil/spark plug, in the same way as checking magneto ignition - to verify that 1. both ignition circuits are working and to deduct spark gap condition from the RPM loss.
You are not supposed to fly in anything “weather” with this license-free aero-moped, a compass would only encourage you to do stupid things with it, like trying to fly from A to B without clear sight of B from A. 
I found at least the Asobo model of it may have a few dangerous traits, like under circumstances starting to fly unexpectedly before the control surfaces (particularly the ailerons) start to do anything useful, and both wings are not necessarily starting to fly at the same time. Getting used to it and trying what it does in slow flight may lead to a few restarts equivalent to fixing the bent $15,000 folding chair airframe and mending fractured bones.