The reason you soak potatoes in water before you fry the potatoes is to prevent the potatoes or fries from sticking together in the deep fryer.
Potatoes contain starch which causes the potatoes or fries to stick when frying them.
So when you remove the starch by soaking the potatoes you prevent the potatoes or fries from sticking to each other.
Also if you need to avoid starch it's a good idea to soak the potatoes in water before cooking or frying them so they'll be less starchy.
Then you can have your potatoes even when you're needing to avoid starchy foods.
Removing the starch from the potatoes also helps to maximize the crispiness of the fries or potatoes.