A couple of reasons.
I like having as many fruits/veggies in my diet as I can. I've read that potatoes cause large spikes in blood sugar levels, and thus I assume, insulin as well, making me think they might be appropriate post-workout -- but after reading a couple Berardi articles, I'm afraid I might be wrong in doing this. That is why I am posting now.
When I put a boiled red potato into my shake along with two scoops chocolate whey, one banana, 30g dextrose, and one cup milk, it tastes like an ice cream milk shake. The potato really transforms the texture and taste, and provides around 30g of what I hope are appropriate post-workout carbs.