In old cars, to engage gears people had to roughly synchronize the speed of gearbox with that of the driving wheels. They had no choice but to double clutch to shift gears.
A synchromesh, or synchro for short, is the solution to this problem. Synchros are devices that allow friction to happen between the meshes before they actually engage, forcing them to synchronize.
This technique helps reducing a lot of stress on the gearbox.
Some folks claim they can shift gears without using the clutch, but this is almost impossible without causing at least some damage. You will need to have super human throttle control and spot-on timing if youd like to try it. If your timing is not perfectly in harmony with your RPMs, the synchromesh will sustain incredible stress that causes extreme wear to these components.
Nowadays its really not necessary anymore to double clutch, but it can be fun to learn, and can be a real saver if you need to drive your car home with a broken clutch!