As mentioned, Insider suggests swapping in water for cream and milk. While Insider does not elaborate on why water is an ingredient worth considering, The New York Times explains that reintroducing the water used to boil the potatoes also helps making the mash fluffier and creamier. In fact, Little House Living quotes this advice from the Little House on the Prairie series: “Leave a very little of the boiling water in, and after you mash them, beat them extra hard with a big spoon.”
Little House Living goes on to explain that when mixed with water, potato starch develops a consistency similar to milk or water mixed with flour. Other uses for potato water include substituting flour, thickening soups, and adding a different texture to your bread. However, water is definitely portrayed as a substitute for when you don’t have milk. This is seen in the Little House recipe for mashed potatoes which simply assumes you have milk or butter at hand that you will use. However, if you don’t like the flavor of alternative milks, water and vegan butter can be combined for a fluffy and creamy vegan mashed potato.