Buttermilk is a classic ingredient in biscuits because it is fermented! Meaning it's full of tanginess from the fermentation so not only is it providing yummy flavor, but that acid is what is needed to work with the leaveners in the dough. If you don't have buttermilk, use whole milk with the juice of a lemon or a 1/2 tsp of white vinegar. Let it sit for a few minutes until you notice the milk starting to get thicker, then it's ready to use.