I think it's quite simple. For as long as I can remember they'd use cooling gills around the cockpit for circuits where temperature is an issue. Since last year they had to place the halo mountings around there, they started using the back of the halo to vent the air out, since that surface will create drag anyways.
It's not there to pull air from the nose, it's not there to energize the flow over the sidepods, it's not there to blow the rear wing.
It's there to keep the engine from overheating and blowing up.
Mercedes already did this last year, it's not like this is the latest and greatest innovation.