TL;DR
Automatically fill ghosts under your cursor when holding ctrl+LMB/shift+LMB or (less convenient) Q+LMB, even while running.What ?
It is the same thing that you can already do via using Pipette on ghosts, but continuous and thus much-much quicker. Also Pipette-from-ghost auto-centers building to cursor (instead of it being on the ghost's place) and therefore is not optimized for filling multicell building ghosts.Why ?
To make early game much faster in a semi-realistical way even without introducing early robots.It's not really fun to manually fill in periodical structures like smelters over and over again especially in MP which restarts often. With this method building an early smelter array is just around six straight wzooms through blueprint. Less time on boring things - more power to go create something interesting. Also it is a more flexible and controllable way of filling (in comparison to fill-everything-in range model).
It wasn't really that needed until 0.15 because only in 0.15 there are early BP.
Robots still give you a very sound advantage later on because with robots you don't need to have stuff in your inventory and also you can place a BP and just go do sth else while stuff is being produced and placed.