When making mashed potatoes you don't have to soak them before cooking them but the reason people soak potatoes before cooking them and making mashed potatoes is the soaking of the potatoes overnight removes the starch from the potatoes.
Removal of the starch by soaking the potatoes helps prevent the mashed potatoes from turning into a paste.
What I do to make mashed potatoes is get a bowl of water and then slice the potatoes up into slices and then let them soak overnight in the water.
Then I drain the water and then add some fresh water and then cook the potatoes until they become soft enough to smash or use a hand mixer to mash them up.
The soaking helps make mashed potatoes much better but you're not required to soak the potatoes overnight.