At the moment, trains coming from Yorkshire approach Piccadilly from the north side, park up, let passengers on and off, then reverse back out of the station and go to the airport on the south side of the tracks. This constant crossing of the tracks from the north side to the south side causes MAJOR delays right across the whole of the North of England.
When this new piece of track (which by the way will be on a viaduct in Salford, so no huge diesel engines on the street or level crossings [sheesh what an idiot!]) is put in place, trains from Yorkshire and beyond will be able to come into Manchester through Victoria station, then Salford Central, then over the new viaduct onto the tracks to Deansgate, Oxford road, and finally Piccadilly, where the train will already be on the south side of the tracks and pointing in the right direction to get to the airport, so no conflicting movements required. All of this also means that more places will be directly linked to the airport, like for example Bradford, and closer to home, Rochdale. It means that there will be loads more capacity too, so more trains (which any regular user will know are badly needed) can run more frequently and with shorter journey times on a range of routes across the North.