Engine power enables a vehicle to get moving and to continue moving. To transfer the engine power into moving force an equal amount of traction is needed - preferably much more traction than torque .
Traction is the resistance or friction happening between each tire and the ground surface.
The other two tires have traction as well, but they are just rolling along, in rear wheel drive vehicles they are used to steer the vehicle. If more torque is applied than there is traction available, the 2 tires will break lose and start spinning. In 4WD power is divided towards 4 tires. Because much more torque is needed to move the vehicle up a steep grade, and only when the torque load is spread out over 4 tires instead of 2 tires it is supported by sufficien traction. When we combine more torque with less traction we'll get a spinning wheel.