Traction and to keep the front or rear end from lifting off the ground. Front end will lift with a lot of weight on the three point hitch and rear will lift with a lot of weight on a loader.
I've had the front end lift up many times on my little tractor. Sometimes it won't quite lift but I won't be able to steer because there's not enough weight on the front tires. One of these days I'll make a tray to hang on the front bumper that will hold 4x8x16 solid concrete blocks.
And of course DIY wheel weights have been made from concrete.
Cast iron wheel weights on a Ford. Looks like a single set but they can be stacked to the point where they stick out past the tires. They make special smooth versions called orchard weights that won't snag the fruit bushes/trees.
Front weighs are aka suitcase weights
