The double clutch technique is an advanced follow up on the heel toe technique. It asks for precision shifting, but isnt required for racing.
The heel toe technique removes excessive and unnecessary loads from the clutch. But even if the heel toe is performed and the clutch engages, the gearbox still needs to synchronize.
The meshes in the gearbox need to synchronize with the vehicles speed, i.e. the drive wheels. Otherwise, the dog teeth on the meshes fail to engage and make a nasty grinding noise.
Now if you put the car into neutral and let the clutch come up (so that the clutch is connected to the gearbox), you can control the speed of the meshes with the throttle. If you double clutch, all you do is adding an extra step to the heel and toe technique.