Calcium chloride helps standardise natural variation in milk. Different feeding conditions for the cows throughout the year mean that the composition of milk changes. If you are buying milk in a supermarket it has been standardised so it is within a smaller range of variation.
Raw milk doesn't have the standardisation process so we strongly recommend that you add calcium chloride.
It's also advisable to use when the milk isn't as fresh.
Calcium chloride helps provide more consistency in your cheesemaking results.