Simply click the noteblock on the left to activate it. It can deliver up to 20 stacks of an item at once by destroying the chest minecart (the items are dropped on the activator rail on the left).
You can set the batch size with the number of items in the hopper clock at the bottom. One item there corresponds to 4 items in the batch.
I use them in my base for a quick refill of either enderpearls, rockets or food. I suppose that it could also be used to give a bunch of emeralds in a trading hall, or some ammo in a minigame, etc…
by No-Letterhead8398