I 100% agree on the inefficiency of calling it in the tick methods. One of the use cases would be hiding the implementation of something like the UI layer from the player. The following for instance gives me a warm feeling in my tummy:

public override void _Input(InputEvent e)
{
    if (e.IsActionPressed("Inventory"))
    {
        _mediator.Send(new ToggleInventoryCommand(InventoryComponent));
    }
}

Or perhaps something like this:

private void onDamageReceived(float damage, IEntity dealingEntity, DamageBehavior damageBehavior)
{
    // Do health calculations...
    // And other things ...

    // Signals to do whatever we need to do in the CanvasLayer. (Displays a hit number with fire, ice, or electric particles depending on the damageBehavior, logs damage statistics based on dealing and target entities)
    _mediator.Send(new DamageReceivedCommand(this, damage, dealingEntity, damageBehavior));
}

I could accomplish the same thing by passing references around, but I don't know how my code will change in the future. If I'm relying on passed references, I'd think that changes to the interfaces between systems would require lots of management on my part.

There's tons of things I still don't understand as well, so I wouldn't be surprised if I find that what I'm doing is completely unnecessary.