Because the mixture cools it down more quickly so it is ready to drink and tastes nicer. Also I can't be sure to get the right amount of cream if I just add a dash. This way if I make up enough for a day or so in a jug I know exactly how many carbs I am having. I'm not usually this fussy but I have only been doing keto for a month so I need to measure to make sure I am going to stay in ketosis.As tea contains water why not just use less cream?