Potatoes are amazing. They grow easily in a wide variety of soil and produce food which has every nutrient we need except for a complete protein. And, they're a vegetable, so you can tell someone you're eating your vegetables and then go to McDonald's for some frenchy fries
Cooking potatoes with skin is still somewhat more nutritious than cooking them without (unless you're keeping the water for something else, some use it for stuff like pasta)
There are several essential amino acids we need to survive but don't produce in our bodies. No plant has all of those amino acids, but every animal product does. You can get complete plant protein by consuming multiple different plants, so if a vegan has a varied diet they're usually fine.