Adds Ballast and, it isn't corrosive like Chloride (used extensively in ND tractor tires), and doesn't freeze like water (used extensively in FL tractor tires where temps don't get cold enough long enough to freeze water in a tire).
Personally I'm a fan of the water routine, but it tends to make your tractor jump up and down at speed after a few days of seriously cold weather. Much easier to work with, screw a $5 adapter on the end of your water hose and fill. Chloride is some nasty Sugar, Honey, Ice Tea to work with, hard on skin, hard on leather boots and gloves, hard on anything metal, hard on concrete... Chloride is however a pound or two heavier than water, which is a pound or so heavier than oil per gallon, where ballast weight is critical.
Set your valve stem at 10 o:clock or 2 o:clock to fill, you have to save a little room for air. Air is much easier to compress, and more forgiving than fluids when compressed.