When you soak potatoes in water before you cook the potatoes the potatoes can sit in the water for up to 12 hours before you cook the potatoes.
Usually soaking the potatoes in water for 7 to 8 hours is enough time before you cook the potatoes.
Soaking the potatoes in the water before cooking them helps to remove starch and helps prevent the potatoes such as fries from sticking together when you fry them.
It also helps make the potatoes or fries more crispy when you fry them after soaking them in water.
You're not required to soak potatoes in water before frying them but doing so makes the potatoes or fries turn out much better.