It's for weight. With my bush hog off I can't lift very much with the forks without the rear tires lifting on my little tractor. On my big tractor I use on the job I have both rear tires filled and a counter balance on the 3 point made of 4 sticks of railroad iron. At around 3,000 lbs it will still start to lift the rear up.
I fill mine by rolling the valve stem to the top. Block or jack it up. Take the valve core out. I put the antifreeze in a pump up sprayer and spray it in. It takes a little bit but it works. Once I have the right amount of antifreeze in I fill it to the top of the rim with water. Tractor Supply or any other tractor place sells a small nozzle that screws on the water hose and goes on the valve stem. It has a pressure valve that lets air out as water goes in. The one I have is made by Slime, the same company that makes the green slime for tires. Once the water is at the top of the wheel remove the nozzle and install the valve core. Air the tire back up as normal.