Look at it this way. All tractors have one wheel drive. Then when you need more traction, you push down on the lever at your feet the locks the rear differential and now you have 2 wheel drive. A so called 4 wheel drive tractor is actually 2 wheel drive all the time, so when you need more traction, you push down on that lever to lock the rear differential, and now you have 3 wheel drive.
It's a significant improvement. I have both types of tractor and it's night and day better getting around in poor conditions with three tires working over 2. In dry conditions, and where you don't really need the most traction possible, 2 wheel drive works fine. I just learned years ago to never take any chances with the 2 wheel drive tractor after it rains. I make sure everything is rock solid dry first!!!