Pressurises the system and gets the pad to the face of the rotor. It makes a huge difference.
You know when you go to press the pedal, it's going to be rock hard. Which it is. If you don't do it, the pedal will travel a long way before the car really starts to pull up. This way, you have total confidence and it pulls up straight away.
It's just a touch, not enough to slow the car at all.
Sandown Turn 1,5 and Winton Turn 1(11) get the same treatment.
Most touring car drivers do the same thing, well that is where I stole it from.
I don't even think about it anymore, just natural.