Everyone should watch this video!!

Before the introduction of transmission synchronizers (in the 1920s) and helical cut gears, double clutching was a technique required to prevent damage to an automobile's gear system (except for cars like the Model T with a planetary gearbox). Due to the difficulty and most often unnecessary redundancy involved in learning the technique, coupled with the advent of synchronized gearing systems, it has largely fallen into disuse. However, drivers of large trucks often use the double clutching technique when unable to keep the transmission unloaded during shifting, as large vehicles are usually equipped with older, simpler and more durable unsynchronized gearboxes.

Can't say I foresee myself needing this skill anytime soon. What type of car/truck has these?