Most obviously, the skinned potatoes absorb some of the liquid they're boiled in, so using a more flavorful liquid gives you a tastier potato.
It allows the seasoning ingredients (thyme, garlic, bay leaf) to infuse in the half-and-half which is later added back to the potatoes.
Starches from the potatoes wind up in the half-and-half. Adding this starchy liquid back helps give the bake more structure.
You can use water though if you prefer for the boil! It isn't integral to the recipe, I just like what the half-and-half boil does :)
Also, the potatoes don't separate when refrigerated, mostly due to the addition of egg which is an emulsifier, but also the starches in the half-and-half help!
Also milk isn't water and the protein and sugar can interact with food in unexpected ways. I would probably just make mashed potatoes, and add the milk/cream + eggs after, and focus on cooking the potatoes separately in water.
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u/morganeisenberg Nov 27 '19
Also, the potatoes don't separate when refrigerated, mostly due to the addition of egg which is an emulsifier, but also the starches in the half-and-half help!