To put it simply Kort...I;'m sure many Canon users know this!!!

MaIn focus shutter/button uses both focus/ exposes the frame!.

Back  focus does what it says!!!...only focuses!
Only till you PRESS the 'shutter' does it expose the frame!
Damn sight faster in focusing!!!
Ask any Pro tog :-)
More keepers than 'loosers'!

As I mentioned, not a new thing, but it does take time to re-adjust
as one needs two fingers to use!

FYI, its CF (Custom Function(CF)...4.3

imo..why not use the functions offered if its beneficial :-)