In 2WD the traction (resistance) of only two wheels is used for forward movement. The other two tires have traction as well, but they are just rolling along. In rear wheel drive vehicles the front tires' traction is used to steer the vehicle. If more torque is applied than there is traction available, the 2 tires will break lose and start spinning. Once the tires are spinning, we say "we lost traction". So, if in need of more power/torque to move more weight or to go faster more traction is needed.
That can be achieved by sending some of the torque to the other two wheels (4WD) and using their traction as well. So, in 2WD torque is equally applied on 2 tires and the traction of those two tires has the burden of supporting the engine's power - each powered tire has deal with 50% of the available torque.
In 4WD torque is directed towards 4 tires. In 4WD the traction at each powered tire has to deal only with 25% of the torque created by engine, transmission, transfer case, and axle. Since each tire in 4WD has to carry a much smaller torque load (25% instead of 50%), it is much much less likely for the tires to break lose. That is why a 4WD can climb much steeper grades than a 2WD.
4WD was invented to use the traction of all 4 tires to either move more weight or drive on surfaces with marginal traction, or both without wheel spin.
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