Tanks and tank destroyers were/are fighting vehicles. They are big, heavy, burn a lot of gas, make a lot of noise and wear out fast so you don't send them meandering around the countryside looking for trouble. For that you have a wheeled vehicle like an armoured car. They can do reconnaisance for enemy outposts, road blocks or other strong points. If they come under fire they have more maueverability and speed which gives them a chance, but not a guarantee, to get away. It's a nasty job being out in front but somebody's got to do it.