Imaginary lines linking the points where the tractor tires touch the ground are called?

The imaginary lines that link the points that you mentioned are called Stability baselines

They are important because of the gravity point of the tractor since the tractor is much taller than normal cars and can easily flip when moving left or right if the driver is not careful due to inertia and the center of gravity. They are made like this because of the job that they have to do.