Drivers usually set their seats to suit their own comfort levels in the kart. But your kart seat is such a crucial part of your chassis and handling that you’ll get from your kart. Your seat will hold the heaviest weight (the driver) in the kart, making it the most influential factor when it comes to your centre of gravity.
Seats & position can be used as an effective setup technique when trying to find the right balance for your kart. You are changing how your weight is distributed to each of the four tyres.