Aside from the frame rails of the chassis, the seat is the most influential component in determining the overall handling of a kart. Based on the design of the seat the Center of Gravity can be altered to better suit the driver's height and weight. Different seat stiffness rates will change the torsion of the frame itself affecting how the inside rear wheel lifts through turns.
Because of the myriad of variables involved, there is no one correct seat for every driver. The one constant is that a seat must be snug fitting to help protect driver. A driver in a seat that is too large will have a greater chance of injury.