For vehicles with selectable four-wheel drive
you can be using free-wheel hub
to shut down the entire drive train (drive shafts, differential)
in 2WD mode.

Advantage: 
Less wear on the impulse-cord and the front-tires,
easier driving and less fuel-consumption
because of smaller roll-resistance

The manual free-wheel-hubs have the additional advantage, that it lets you know,
whether they are switched on or not.

Important:
Always control both sides and switch on in time.