4WD has been invented around 1900
to allow carrying more weight. Since 4WD also creates better driving stability it has found its way into many modern cars as a safety feature.
The engine creates the rotational force "torque" to move the vehicle. The counter force "traction" allows torque to create forward movement.
If the counter force traction is less than the amount of torque created by the driver's right foot - the wheels break lose and spin. Forward movement will cease.
If two wheels are used to bring torque and traction together the available traction at the two wheels will limit the amount of torque than can be applied.
Since heavy trucks will need more torque to be moved, they sometimes require more traction than 2 wheels can provide.
That lead to the creation of 4WD. Torque is now sent to 2 additional wheels, which provide 100% more traction. 100% more torque can be applied without slipping and spinning wheels.