But why are they building a platform in the middle of the tracks when a perfectly good one already exists directly behind it?
It's part of the grand overhaul currently underway at the station, which includes a renovation of the arch over Fourth Avenue; I imagine, though have not confirmed, that this platform will be in temporary use while that side of the arch is worked on. Personally, I hope above all else that they remove the light-barring metal coverings that obscure what were once windows, stopping all light from coming into a station that once had it streaming in. (Taking down those horrendous billboards over Fourth Avenue wouldn't hurt, either.) That train station is one of the nicest in the system—if the MTA would clean it up a bit and restore it to its former glory, just barely visible beneath decades of accumulated grime.