Wheels are not necessary for the functioning of in-game vehicles, but they greatly help with speed, stability, and manoeuvrability.
Wheels have a health status, and can be shot at, which will pop them.
Stand next to the vehicle and open it's inventory, slide the wheel into it's respective slot.
Wheels have a large weight, and it's often impossible to carry more than two at a time.
Loot Spawn Edit
Wheels spawn in hardware stores, garages, warehouses, gas stations, boxes, and most technical buildings.