Yep I only became aware of this during my visit earlier on this week, so it is new to me as well and I have not used it yet.
You make your booking Online, then when you arrive at Immigration you go straight to 'Window 3' the Right Hand Side window.
The person at Window 3 checks your papers and when correct gives you your queue number to see Immigration Officer.
Where this system really helps is if there is a random turn up, you will get priority because of your booking time.
I would assume this way Immigration know how many planned people are coming in any one day and can plan for that!!
Who knows... If this is a trial and successful - it may become the only and new normal way to visit!