Why is it recommended to mix yogurt starter with a small amount of milk before adding to the entire batch?
Dry powders are easier to mix if you make a slurry first with a small amount of liquid and then mix the slurry in.

If you skip this step you will have clumps of dry powder floating on top of the milk and it will take a lot more effort to mix in.

When you are using yoghurt as a starter for a new batch this step is not necessary and the starter can just be mixed in directly.