For example the minimum required weight of a Formula 3 car is 565kg, including the driver. On average the car on its own weighs just over 490kg meaning that a driver over 75kg would just make the weight limit. So for a driver like me, who ways 65kg with equipment, the team would have to add some ballast to the car to make the minimum weight.

The advantage of being a lower weight is that the ballast on the car can be moved around, generally the idea is to put it as low as possible. This lowers the centre of gravity generally improving handling. The ballast can also be distributed to the front or to the rear, This can also manipulate the handling of the car quite a bit. So the more weight you are able to move around the better.
As a driver you have to manage your weight without compromising your fitness and strength. At some high downforce circuits like Zandvoort and Silverstone it can become really physical through the high speed corners. So you need to make sure you are fit and strong enough not to struggle on these types of circuit.