Doodle, mainly for sidewall stiffness/load carrying capacity. A 10 ply tire doesn't flex as much as a 8 or 6 ply tire, therefore, weight carrying capacity and trailer towing can be optimized. The downside is that without much flex, the truck doesn't ride as nice. Look at the load ratings for the tires you are looking to buy, and if you are pulling/hauling heavy loads, try to stay in the same load range (Your current tire will be "E" rated. My 285/75r16 tires have a "D" rating, however, they are only eight ply. Load capacity is about the same, but you can tell the difference if you hook a trailer (heavy one) vs. the stock tires.
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