Simple, they are wheels... but with some adjustments...
Wheels greatly affect robot's movement. Some wheels even have extra gears to increase combat ability, and to make your automobile more badass. Players will gain an advantage in combat when they have a good mix of wheels and other robot parts.
Stats for Wheels:
Health: Bonus HP for the robot.
Magic: Bonus stats which appear proportionally based on the wheels' rarity.
Tiers: Same with Body, Wheels have tiers that determine their robot's stats.