One reason is in QQuickDeliveryAgentPrivate::deliverPointerEvent():

    if (event->isBeginEvent()) {
        ensureDeviceConnected(event->pointingDevice());
        if (!deliverPressOrReleaseEvent(event))
            event->setAccepted(false);
    }

deliverPressOrReleaseEvent() returned false after trying to click a Switch.

The ending state often doesn't matter in practice; but QGuiApplicationPrivate::processMouseEvent() synthesizes touch events if AA_SynthesizeTouchForUnhandledMouseEvents is set (which ought to be rare); and maybe we actually want to come up with a way to detect that a mouse press (or touch press) was completely un-handled. And maybe some QPA plugins could care: WindowSystemEvent::eventAccepted is set according to the ending accepted state of the QMouseEvent, and as long as delivery is synchronous, the reason is so that the platform plugin can see the result: was the event delivered successfully or not.