When should SignalSpy be used in QML?

Question

I'm seeing an issue in my QML animation, where very rarely, a state transition animation does not happen. I was searching online why that might be and came across SignalSpy. How and when should someone use it? The example is confusing, I thought it was certain that every time you click a button, the clicked signal would be emitted, why do we need to test this? I was just looking for an explanation that goes beyond the few lines mentioned in the documentation here: https://doc.qt.io/qt-6/qml-qttest-signalspy.html